variable.html
No OneTemporary
Actions

Subscribers

None

File Metadata

Created: Fri, Aug 16, 13:08

variable.html
View Options

	<HTML>
	<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
	</CENTER>






	<HR>

	<H3>variable command
	</H3>
	<P><B>Syntax:</B>
	</P>
	<PRE>variable name style args ...
	</PRE>
	<UL><LI>name = name of variable to define

	<LI>style = <I>delete</I> or <I>index</I> or <I>loop</I> or <I>world</I> or <I>universe</I> or
	<I>uloop</I> or <I>string</I> or <I>format</I> or <I>getenv</I> or <I>file</I> or <I>atomfile</I> or <I>python</I> or <I>equal</I> or <I>atom</I>

	<PRE> <I>delete</I> = no args
	<I>index</I> args = one or more strings
	<I>loop</I> args = N
	N = integer size of loop, loop from 1 to N inclusive
	<I>loop</I> args = N pad
	N = integer size of loop, loop from 1 to N inclusive
	pad = all values will be same length, e.g. 001, 002, ..., 100
	<I>loop</I> args = N1 N2
	N1,N2 = loop from N1 to N2 inclusive
	<I>loop</I> args = N1 N2 pad
	N1,N2 = loop from N1 to N2 inclusive
	pad = all values will be same length, e.g. 050, 051, ..., 100
	<I>world</I> args = one string for each partition of processors
	<I>universe</I> args = one or more strings
	<I>uloop</I> args = N
	N = integer size of loop
	<I>uloop</I> args = N pad
	N = integer size of loop
	pad = all values will be same length, e.g. 001, 002, ..., 100
	<I>string</I> arg = one string
	<I>format</I> args = vname fstr
	vname = name of equal-style variable to evaluate
	fstr = C-style format string
	<I>getenv</I> arg = one string
	<I>file</I> arg = filename
	<I>atomfile</I> arg = filename
	<I>python</I> arg = function
	<I>equal</I> or <I>atom</I> args = one formula containing numbers, thermo keywords, math operations, group functions, atom values and vectors, compute/fix/variable references
	numbers = 0.0, 100, -5.4, 2.8e-4, etc
	constants = PI, version, on, off, true, false, yes, no
	thermo keywords = vol, ke, press, etc from <A HREF = "thermo_style.html">thermo_style</A>
	math operators = (), -x, x+y, x-y, x*y, x/y, x^y, x%y,
	x == y, x != y, x &lt y, x &lt= y, x &gt y, x &gt= y, x && y, x \|\| y, !x
	math functions = sqrt(x), exp(x), ln(x), log(x), abs(x),
	sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x),
	random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x)
	ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z),
	stride(x,y,z), stride2(x,y,z,a,b,c),
	vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z)
	group functions = count(group), mass(group), charge(group),
	xcm(group,dim), vcm(group,dim), fcm(group,dim),
	bound(group,dir), gyration(group), ke(group),
	angmom(group,dim), torque(group,dim),
	inertia(group,dimdim), omega(group,dim)
	region functions = count(group,region), mass(group,region), charge(group,region),
	xcm(group,dim,region), vcm(group,dim,region), fcm(group,dim,region),
	bound(group,dir,region), gyration(group,region), ke(group,reigon),
	angmom(group,dim,region), torque(group,dim,region),
	inertia(group,dimdim,region), omega(group,dim,region)
	special functions = sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x)
	feature functions = is_active(category,feature,exact), is_defined(category,id,exact)
	atom value = id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i]
	atom vector = id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q
	compute references = c_ID, c_ID[i], c_ID[i][j]
	fix references = f_ID, f_ID[i], f_ID[i][j]
	variable references = v_name, v_name[i]
	</PRE>

	</UL>
	<P><B>Examples:</B>
	</P>
	<PRE>variable x index run1 run2 run3 run4 run5 run6 run7 run8
	variable LoopVar loop $n
	variable beta equal temp/3.0
	variable b1 equal x[234]+0.5*vol
	variable b1 equal "x[234] + 0.5*vol"
	variable b equal xcm(mol1,x)/2.0
	variable b equal c_myTemp
	variable b atom x*y/vol
	variable foo string myfile
	variable myPy python increase
	variable f file values.txt
	variable temp world 300.0 310.0 320.0 ${Tfinal}
	variable x universe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
	variable x uloop 15 pad
	variable str format x %.6g
	variable x delete
	</PRE>
	<P><B>Description:</B>
	</P>
	<P>This command assigns one or more strings to a variable name for
	evaluation later in the input script or during a simulation.
	</P>
	<P>Variables can thus be useful in several contexts. A variable can be
	defined and then referenced elsewhere in an input script to become
	part of a new input command. For variable styles that store multiple
	strings, the <A HREF = "next.html">next</A> command can be used to increment which
	string is assigned to the variable. Variables of style <I>equal</I> store
	a formula which when evaluated produces a single numeric value which
	can be output either directly (see the <A HREF = "print.html">print</A>, <A HREF = "fix_print.html">fix
	print</A>, and <A HREF = "run.html">run every</A> commands) or as part
	of thermodynamic output (see the <A HREF = "thermo_style.html">thermo_style</A>
	command), or used as input to an averaging fix (see the <A HREF = "fix_ave_time.html">fix
	ave/time</A> command). Variables of style <I>atom</I> store
	a formula which when evaluated produces one numeric value per atom
	which can be output to a dump file (see the <A HREF = "dump.html">dump custom</A>
	command) or used as input to an averaging fix (see the <A HREF = "fix_ave_spatial.html">fix
	ave/spatial</A> and <A HREF = "fix_ave_atom.html">fix ave/atom</A>
	commands). Variables of style <I>atomfile</I> can be used anywhere in an
	input script that atom-style variables are used; they get their
	per-atom values from a file rather than from a formula. Variables can
	be hooked to Python functions using code you provide, so that the
	variable gets its value from the evaluation of the Python code.
	</P>
	<P>IMPORTANT NOTE: As discussed in <A HREF = "Section_commands.html#cmd_2">Section 3.2</A>
	of the manual, an input script can use "immediate" variables, specified
	as $(formula) with parenthesis, where the formula has the same syntax
	as equal-style variables described on this page. This is a convenient
	way to evaluate a formula immediately without using the variable command
	to define a named variable and then evaluate that variable. See below
	for a more detailed discussion of this feature.
	</P>
	<P>In the discussion that follows, the "name" of the variable is the
	arbitrary string that is the 1st argument in the variable command.
	This name can only contain alphanumeric characters and underscores.
	The "string" is one or more of the subsequent arguments. The "string"
	can be simple text as in the 1st example above, it can contain other
	variables as in the 2nd example, or it can be a formula as in the 3rd
	example. The "value" is the numeric quantity resulting from
	evaluation of the string. Note that the same string can generate
	different values when it is evaluated at different times during a
	simulation.
	</P>
	<P>IMPORTANT NOTE: When the input script line is encountered that defines
	a variable of style <I>equal</I> or <I>atom</I> or <I>python</I> that contains a
	formula or Python code, the formula is NOT immediately evaluated.
	It will be evaluated every time when the variable is <B>used</B> instead.
	If you simply want to evaluate a formula in place you can use as
	so-called. See the section below about "Immediate Evaluation
	of Variables" for more details on the topic. This is also true of
	a <I>format</I> style variable since it evaluates another variable when
	it is invoked.
	</P>
	<P>IMPORTANT NOTE: Variables of style <I>equal</I> and <I>atom</I> can be used as
	inputs to various other LAMMPS commands which evaluate their formulas
	as needed, e.g. at different timesteps during a <A HREF = "run.html">run</A>.
	Variables of style <I>python</I> can be used in place of an equal-style
	variable so long as the associated Python function, as defined by the
	<A HREF = "python.html">python</A> command, returns a numeric value. Thus any
	command that states it can use an equal-style variable as an argument,
	can also use such a python-style variable. This means that when the
	LAMMPS command evaluates the variable, the Python function will be
	executed.
	</P>
	<P>IMPORTANT NOTE: When a variable command is encountered in the input
	script and the variable name has already been specified, the command
	is ignored. This means variables can NOT be re-defined in an input
	script (with two exceptions, read further). This is to allow an input
	script to be processed multiple times without resetting the variables;
	see the <A HREF = "jump.html">jump</A> or <A HREF = "include.html">include</A> commands. It also
	means that using the <A HREF = "Section_start.html#start_7">command-line switch</A>
	-var will override a corresponding index variable setting in the input
	script.
	</P>
	<P>There are two exceptions to this rule. First, variables of style
	<I>string</I>, <I>getenv</I>, <I>equal</I>, <I>atom</I>, and <I>python</I> ARE redefined each
	time the command is encountered. This allows these style of variables
	to be redefined multiple times in an input script. In a loop, this
	means the formula associated with an <I>equal</I> or <I>atom</I> style variable
	can change if it contains a substitution for another variable, e.g. $x
	or v_x.
	</P>
	<P>Second, as described below, if a variable is iterated on to the end of
	its list of strings via the <A HREF = "next.html">next</A> command, it is removed
	from the list of active variables, and is thus available to be
	re-defined in a subsequent variable command. The <I>delete</I> style does
	the same thing.
	</P>
	<HR>

	<P><A HREF = "Section_commands.html#cmd_2">This section</A> of the manual explains how
	occurrences of a variable name in an input script line are replaced by
	the variable's string. The variable name can be referenced as $x if
	the name "x" is a single character, or as ${LoopVar} if the name
	"LoopVar" is one or more characters.
	</P>
	<P>As described below, for variable styles <I>index</I>, <I>loop</I>, <I>file</I>,
	<I>universe</I>, and <I>uloop</I>, which string is assigned to a variable can be
	incremented via the <A HREF = "next.html">next</A> command. When there are no more
	strings to assign, the variable is exhausted and a flag is set that
	causes the next <A HREF = "jump.html">jump</A> command encountered in the input
	script to be skipped. This enables the construction of simple loops
	in the input script that are iterated over and then exited from.
	</P>
	<P>As explained above, an exhausted variable can be re-used in an input
	script. The <I>delete</I> style also removes the variable, the same as if
	it were exhausted, allowing it to be redefined later in the input
	script or when the input script is looped over. This can be useful
	when breaking out of a loop via the <A HREF = "if.html">if</A> and <A HREF = "jump.html">jump</A>
	commands before the variable would become exhausted. For example,
	</P>
	<PRE>label loop
	variable a loop 5
	print "A = $a"
	if "$a > 2" then "jump in.script break"
	next a
	jump in.script loop
	label break
	variable a delete
	</PRE>
	<HR>

	<P>This section describes how all the various variable styles are defined
	and what they store. Except for the <I>equal</I> and <I>atom</I> styles,
	which are explaine in the next section.
	</P>
	<P>Many of the styles store one or more strings. Note that a single
	string can contain spaces (multiple words), if it is enclosed in
	quotes in the variable command. When the variable is substituted for
	in another input script command, its returned string will then be
	interpreted as multiple arguments in the expanded command.
	</P>
	<P>For the <I>index</I> style, one or more strings are specified. Initially,
	the 1st string is assigned to the variable. Each time a
	<A HREF = "next.html">next</A> command is used with the variable name, the next
	string is assigned. All processors assign the same string to the
	variable.
	</P>
	<P><I>Index</I> style variables with a single string value can also be set by
	using the command-line switch -var; see <A HREF = "Section_start.html#start_7">this
	section</A> for details.
	</P>
	<P>The <I>loop</I> style is identical to the <I>index</I> style except that the
	strings are the integers from 1 to N inclusive, if only one argument N
	is specified. This allows generation of a long list of runs
	(e.g. 1000) without having to list N strings in the input script.
	Initially, the string "1" is assigned to the variable. Each time a
	<A HREF = "next.html">next</A> command is used with the variable name, the next
	string ("2", "3", etc) is assigned. All processors assign the same
	string to the variable. The <I>loop</I> style can also be specified with
	two arguments N1 and N2. In this case the loop runs from N1 to N2
	inclusive, and the string N1 is initially assigned to the variable.
	N1 <= N2 and N2 >= 0 is required.
	</P>
	<P>For the <I>world</I> style, one or more strings are specified. There must
	be one string for each processor partition or "world". See <A HREF = "Section_start.html#start_7">this
	section</A> of the manual for information on
	running LAMMPS with multiple partitions via the "-partition"
	command-line switch. This variable command assigns one string to each
	world. All processors in the world are assigned the same string. The
	next command cannot be used with <I>equal</I> style variables, since there
	is only one value per world. This style of variable is useful when
	you wish to run different simulations on different partitions, or when
	performing a parallel tempering simulation (see the
	<A HREF = "temper.html">temper</A> command), to assign different temperatures to
	different partitions.
	</P>
	<P>For the <I>universe</I> style, one or more strings are specified. There
	must be at least as many strings as there are processor partitions or
	"worlds". See <A HREF = "Section_start.html#start_7">this page</A> for information
	on running LAMMPS with multiple partitions via the "-partition"
	command-line switch. This variable command initially assigns one
	string to each world. When a <A HREF = "next.html">next</A> command is encountered
	using this variable, the first processor partition to encounter it, is
	assigned the next available string. This continues until all the
	variable strings are consumed. Thus, this command can be used to run
	50 simulations on 8 processor partitions. The simulations will be run
	one after the other on whatever partition becomes available, until
	they are all finished. <I>Universe</I> style variables are incremented
	using the files "tmp.lammps.variable" and "tmp.lammps.variable.lock"
	which you will see in your directory during such a LAMMPS run.
	</P>
	<P>The <I>uloop</I> style is identical to the <I>universe</I> style except that the
	strings are the integers from 1 to N. This allows generation of long
	list of runs (e.g. 1000) without having to list N strings in the input
	script.
	</P>
	<P>For the <I>string</I> style, a single string is assigned to the variable.
	The only difference between this and using the <I>index</I> style with a
	single string is that a variable with <I>string</I> style can be redefined.
	E.g. by another command later in the input script, or if the script is
	read again in a loop.
	</P>
	<P>For the <I>format</I> style, an equal-style variable is specified along
	with a C-style format string, e.g. "%f" or "%.10g", which must be
	appropriate for formatting a double-precision floating-point value.
	This allows an equal-style variable to be formatted specifically for
	output as a string, e.g. by the <A HREF = "print.html">print</A> command, if the
	default format "%.15g" has too much precision.
	</P>
	<P>For the <I>getenv</I> style, a single string is assigned to the variable
	which should be the name of an environment variable. When the
	variable is evaluated, it returns the value of the environment
	variable, or an empty string if it not defined. This style of
	variable can be used to adapt the behavior of LAMMPS input scripts via
	environment variable settings, or to retrieve information that has
	been previously stored with the <A HREF = "shell.html">shell putenv</A> command.
	Note that because environment variable settings are stored by the
	operating systems, they persist beyond a <A HREF = "clear.html">clear</A> command.
	</P>
	<P>For the <I>file</I> style, a filename is provided which contains a list of
	strings to assign to the variable, one per line. The strings can be
	numeric values if desired. See the discussion of the next() function
	below for equal-style variables, which will convert the string of a
	file-style variable into a numeric value in a formula.
	</P>
	<P>When a file-style variable is defined, the file is opened and the
	string on the first line is read and stored with the variable. This
	means the variable can then be evaluated as many times as desired and
	will return that string. There are two ways to cause the next string
	from the file to be read: use the <A HREF = "next.html">next</A> command or the
	next() function in an equal- or atom-style variable, as discussed
	below.
	</P>
	<P>The rules for formatting the file are as follows. A comment character
	"#" can be used anywhere on a line; text starting with the comment
	character is stripped. Blank lines are skipped. The first "word" of
	a non-blank line, delimited by white space, is the "string" assigned
	to the variable.
	</P>
	<P>For the <I>atomfile</I> style, a filename is provided which contains one or
	more sets of values, to assign on a per-atom basis to the variable.
	The format of the file is described below.
	</P>
	<P>When an atomfile-style variable is defined, the file is opened and the
	first set of per-atom values are read and stored with the variable.
	This means the variable can then be evaluated as many times as desired
	and will return those values. There are two ways to cause the next
	set of per-atom values from the file to be read: use the
	<A HREF = "next.html">next</A> command or the next() function in an atom-style
	variable, as discussed below.
	</P>
	<P>The rules for formatting the file are as follows. Each time a set of
	per-atom values is read, a non-blank line is searched for in the file.
	A comment character "#" can be used anywhere on a line; text starting
	with the comment character is stripped. Blank lines are skipped. The
	first "word" of a non-blank line, delimited by white space, is read as
	the count N of per-atom lines to immediately follow. N can be be the
	total number of atoms in the system, or only a subset. The next N
	lines have the following format
	</P>
	<PRE>ID value
	</PRE>
	<P>where ID is an atom ID and value is the per-atom numeric value that
	will be assigned to that atom. IDs can be listed in any order.
	</P>
	<P>IMPORTANT NOTE: Every time a set of per-atom lines is read, the value
	for all atoms is first set to 0.0. Thus values for atoms whose ID
	does not appear in the set, will remain 0.0.
	</P>
	<P>For the <I>python</I> style a Python function name is provided. This needs
	to match a function name specified in a <A HREF = "python.html">python</A> command
	which returns a value to this variable as defined by its <I>return</I>
	keyword. For exampe these two commands would be self-consistent:
	</P>
	<PRE>variable foo python myMultiply
	python myMultiply return v_foo format f file funcs.py
	</PRE>
	<P>The two commands can appear in either order so long as both are
	specified before the Python function is invoked for the first time.
	</P>
	<P>Each time the variable is evaluated, the associated Python function is
	invoked, and the value it returns is also returned by the variable.
	Since the Python function can use other LAMMPS variables as input, or
	query interal LAMMPS quantities to perform its computation, this means
	the variable can return a different value each time it is evaluated.
	</P>
	<P>The type of value stored in the variable is determined by the <I>format</I>
	keyword of the <A HREF = "python.html">python</A> command. It can be an integer
	(i), floating point (f), or string (s) value. As mentioned above, if
	it is a numeric value (integer or floating point), then the
	python-style variable can be used in place of an equal-style variable
	anywhere in an input script, e.g. as an argument to another command
	that allows for equal-style variables.
	</P>
	<HR>

	<P>For the <I>equal</I> and <I>atom</I> styles, a single string is specified which
	represents a formula that will be evaluated afresh each time the
	variable is used. If you want spaces in the string, enclose it in
	double quotes so the parser will treat it as a single argument. For
	<I>equal</I> style variables the formula computes a scalar quantity, which
	becomes the value of the variable whenever it is evaluated. For
	<I>atom</I> style variables the formula computes one quantity for each
	atom whenever it is evaluated.
	</P>
	<P>Note that <I>equal</I> and <I>atom</I> variables can produce different values at
	different stages of the input script or at different times during a
	run. For example, if an <I>equal</I> variable is used in a <A HREF = "fix_print.html">fix
	print</A> command, different values could be printed each
	timestep it was invoked. If you want a variable to be evaluated
	immediately, so that the result is stored by the variable instead of
	the string, see the section below on "Immediate Evaluation of
	Variables".
	</P>
	<P>The next command cannot be used with <I>equal</I> or <I>atom</I> style
	variables, since there is only one string.
	</P>
	<P>The formula for an <I>equal</I> or <I>atom</I> variable can contain a variety
	of quantities. The syntax for each kind of quantity is simple, but
	multiple quantities can be nested and combined in various ways to
	build up formulas of arbitrary complexity. For example, this is a
	valid (though strange) variable formula:
	</P>
	<PRE>variable x equal "pe + c_MyTemp / vol^(1/3)"
	</PRE>
	<P>Specifically, an formula can contain numbers, thermo keywords, math
	operators, math functions, group functions, region functions, atom
	values, atom vectors, compute references, fix references, and
	references to other variables.
	</P>
	<DIV ALIGN=center><TABLE BORDER=1 >
	<TR><TD >Number</TD><TD > 0.2, 100, 1.0e20, -15.4, etc</TD></TR>
	<TR><TD >Constant</TD><TD > PI, version, on, off, true, false, yes, no</TD></TR>
	<TR><TD >Thermo keywords</TD><TD > vol, pe, ebond, etc</TD></TR>
	<TR><TD >Math operators</TD><TD > (), -x, x+y, x-y, x*y, x/y, x^y, x%y, </TD></TR>
	<TR><TD >Math operators</TD><TD > (), -x, x+y, x-y, x*y, x/y, x^y, x%y, x == y, x != y, x &lt y, x &lt= y, x &gt y, x &gt= y, x && y, x \|\| y, !x</TD></TR>
	<TR><TD >Math functions</TD><TD > sqrt(x), exp(x), ln(x), log(x), abs(x), sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x), random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x), ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z), stride(x,y,z), stride2(x,y,z,a,b,c), vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z)</TD></TR>
	<TR><TD >Group functions</TD><TD > count(ID), mass(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), fcm(ID,dim), bound(ID,dir), gyration(ID), ke(ID), angmom(ID,dim), torque(ID,dim), inertia(ID,dimdim), omega(ID,dim)</TD></TR>
	<TR><TD >Region functions</TD><TD > count(ID,IDR), mass(ID,IDR), charge(ID,IDR), xcm(ID,dim,IDR), vcm(ID,dim,IDR), fcm(ID,dim,IDR), bound(ID,dir,IDR), gyration(ID,IDR), ke(ID,IDR), angmom(ID,dim,IDR), torque(ID,dim,IDR), inertia(ID,dimdim,IDR), omega(ID,dim,IDR)</TD></TR>
	<TR><TD >Special functions</TD><TD > sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x)</TD></TR>
	<TR><TD >Atom values</TD><TD > id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i]</TD></TR>
	<TR><TD >Atom vectors</TD><TD > id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q</TD></TR>
	<TR><TD >Compute references</TD><TD > c_ID, c_ID[i], c_ID[i][j]</TD></TR>
	<TR><TD >Fix references</TD><TD > f_ID, f_ID[i], f_ID[i][j]</TD></TR>
	<TR><TD >Other variables</TD><TD > v_name, v_name[i]
	</TD></TR></TABLE></DIV>

	<HR>

	<P>Most of the formula elements produce a scalar value. A few produce a
	per-atom vector of values. These are the atom vectors, compute
	references that represent a per-atom vector, fix references that
	represent a per-atom vector, and variables that are atom-style
	variables. Math functions that operate on scalar values produce a
	scalar value; math function that operate on per-atom vectors do so
	element-by-element and produce a per-atom vector.
	</P>
	<P>A formula for equal-style variables cannot use any formula element
	that produces a per-atom vector. A formula for an atom-style variable
	can use formula elements that produce either a scalar value or a
	per-atom vector. Atom-style variables are evaluated by other commands
	that define a <A HREF = "group.html">group</A> on which they operate, e.g. a
	<A HREF = "dump.html">dump</A> or <A HREF = "compute.html">compute</A> or <A HREF = "fix.html">fix</A> command.
	When they invoke the atom-style variable, only atoms in the group are
	inlcuded in the formula evaluation. The variable evaluates to 0.0 for
	atoms not in the group.
	</P>
	<P>Constants are set at compile time and cannot be changed. <I>PI</I> will
	return the number 3.14159265358979323846; <I>on</I>, <I>true</I> or <I>yes</I> will
	return 1.0; <I>off</I>, <I>false</I> or <I>no</I> will return 0.0; <I>version</I> will
	return a numeric version code of the current LAMMPS version (e.g.
	version 2 Sep 2015 will return the number 20150902). The corresponding
	value for newer versions of LAMMPS will be larger, for older versions
	of LAMMPS will be smaller. This can be used to have input scripts
	adapt automatically to LAMMPS versions, when non-backwards compatible
	syntax changes are introduced. Here is an illustrative example (which
	will not work, since the <I>version</I> has been introduced more recently):
	</P>
	<PRE>if $(version<20140513) then "communicate vel yes" else "comm_modify vel yes"
	</PRE>
	<P>The thermo keywords allowed in a formula are those defined by the
	<A HREF = "thermo_style.html">thermo_style custom</A> command. Thermo keywords that
	require a <A HREF = "compute.html">compute</A> to calculate their values such as
	"temp" or "press", use computes stored and invoked by the
	<A HREF = "thermo_style.html">thermo_style</A> command. This means that you can
	only use those keywords in a variable if the style you are using with
	the thermo_style command (and the thermo keywords associated with that
	style) also define and use the needed compute. Note that some thermo
	keywords use a compute indirectly to calculate their value (e.g. the
	enthalpy keyword uses temp, pe, and pressure). If a variable is
	evaluated directly in an input script (not during a run), then the
	values accessed by the thermo keyword must be current. See the
	discussion below about "Variable Accuracy".
	</P>
	<HR>

	<H4>Math Operators
	</H4>
	<P>Math operators are written in the usual way, where the "x" and "y" in
	the examples can themselves be arbitrarily complex formulas, as in the
	examples above. In this syntax, "x" and "y" can be scalar values or
	per-atom vectors. For example, "ke/natoms" is the division of two
	scalars, where "vy+vz" is the element-by-element sum of two per-atom
	vectors of y and z velocities.
	</P>
	<P>Operators are evaluated left to right and have the usual C-style
	precedence: unary minus and unary logical NOT operator "!" have the
	highest precedence, exponentiation "^" is next; multiplication and
	division and the modulo operator "%" are next; addition and
	subtraction are next; the 4 relational operators "<", "<=", ">", and
	">=" are next; the two remaining relational operators "==" and "!="
	are next; then the logical AND operator "&&"; and finally the logical
	OR operator "\|\|" has the lowest precedence. Parenthesis can be used
	to group one or more portions of a formula and/or enforce a different
	order of evaluation than what would occur with the default precedence.
	</P>
	<P>IMPORTANT NOTE: Because a unary minus is higher precedence than
	exponentiation, the formula "-2^2" will evaluate to 4, not -4. This
	convention is compatible with some programming languages, but not
	others. As mentioned, this behavior can be easily overridden with
	parenthesis; the formula "-(2^2)" will evaluate to -4.
	</P>
	<P>The 6 relational operators return either a 1.0 or 0.0 depending on
	whether the relationship between x and y is TRUE or FALSE. For
	example the expression x<10.0 in an atom-style variable formula will
	return 1.0 for all atoms whose x-coordinate is less than 10.0, and 0.0
	for the others. The logical AND operator will return 1.0 if both its
	arguments are non-zero, else it returns 0.0. The logical OR operator
	will return 1.0 if either of its arguments is non-zero, else it
	returns 0.0. The logical NOT operator returns 1.0 if its argument is
	0.0, else it returns 0.0.
	</P>
	<P>These relational and logical operators can be used as a masking or
	selection operation in a formula. For example, the number of atoms
	whose properties satifsy one or more criteria could be calculated by
	taking the returned per-atom vector of ones and zeroes and passing it
	to the <A HREF = "compute_reduce.html">compute reduce</A> command.
	</P>
	<HR>

	<H4>Math Functions
	</H4>
	<P>Math functions are specified as keywords followed by one or more
	parenthesized arguments "x", "y", "z", each of which can themselves be
	arbitrarily complex formulas. In this syntax, the arguments can
	represent scalar values or per-atom vectors. In the latter case, the
	math operation is performed on each element of the vector. For
	example, "sqrt(natoms)" is the sqrt() of a scalar, where "sqrt(y*z)"
	yields a per-atom vector with each element being the sqrt() of the
	product of one atom's y and z coordinates.
	</P>
	<P>Most of the math functions perform obvious operations. The ln() is
	the natural log; log() is the base 10 log.
	</P>
	<P>The random(x,y,z) function takes 3 arguments: x = lo, y = hi, and z =
	seed. It generates a uniform random number between lo and hi. The
	normal(x,y,z) function also takes 3 arguments: x = mu, y = sigma, and
	z = seed. It generates a Gaussian variate centered on mu with
	variance sigma^2. In both cases the seed is used the first time the
	internal random number generator is invoked, to initialize it. For
	equal-style variables, every processor uses the same seed so that they
	each generate the same sequence of random numbers. For atom-style
	variables, a unique seed is created for each processor, based on the
	specified seed. This effectively generates a different random number
	for each atom being looped over in the atom-style variable.
	</P>
	<P>IMPORTANT NOTE: Internally, there is just one random number generator
	for all equal-style variables and one for all atom-style variables.
	If you define multiple variables (of each style) which use the
	random() or normal() math functions, then the internal random number
	generators will only be initialized once, which means only one of the
	specified seeds will determine the sequence of generated random
	numbers.
	</P>
	<P>The ceil(), floor(), and round() functions are those in the C math
	library. Ceil() is the smallest integer not less than its argument.
	Floor() if the largest integer not greater than its argument. Round()
	is the nearest integer to its argument.
	</P>
	<P>The ramp(x,y) function uses the current timestep to generate a value
	linearly intepolated between the specified x,y values over the course
	of a run, according to this formula:
	</P>
	<PRE>value = x + (y-x) * (timestep-startstep) / (stopstep-startstep)
	</PRE>
	<P>The run begins on startstep and ends on stopstep. Startstep and
	stopstep can span multiple runs, using the <I>start</I> and <I>stop</I> keywords
	of the <A HREF = "run.html">run</A> command. See the <A HREF = "run.html">run</A> command for
	details of how to do this.
	</P>
	<P>The stagger(x,y) function uses the current timestep to generate a new
	timestep. X,y > 0 and x > y are required. The generated timesteps
	increase in a staggered fashion, as the sequence
	x,x+y,2x,2x+y,3x,3x+y,etc. For any current timestep, the next
	timestep in the sequence is returned. Thus if stagger(1000,100) is
	used in a variable by the <A HREF = "dump_modify.html">dump_modify every</A>
	command, it will generate the sequence of output timesteps:
	</P>
	<PRE>100,1000,1100,2000,2100,3000,etc
	</PRE>
	<P>The logfreq(x,y,z) function uses the current timestep to generate a
	new timestep. X,y,z > 0 and y < z are required. The generated
	timesteps are on a base-z logarithmic scale, starting with x, and the
	y value is how many of the z-1 possible timesteps within one
	logarithmic interval are generated. I.e. the timesteps follow the
	sequence x,2x,3x,...yx,xz,2xz,3xz,...yxz,xz^2,2xz^2,etc. For
	any current timestep, the next timestep in the sequence is returned.
	Thus if logfreq(100,4,10) is used in a variable by the <A HREF = "dump_modify.html">dump_modify
	every</A> command, it will generate this sequence of
	output timesteps:
	</P>
	<PRE>100,200,300,400,1000,2000,3000,4000,10000,20000,etc
	</PRE>
	<P>The logfreq2(x,y,z) function is similar to logfreq, except a single
	logarithmic interval is divided into y equally-spaced timesteps and
	all of them are output. Y < z is not required. Thus, if
	logfreq2(100,18,10) is used in a variable by the <A HREF = "dump_modify.html">dump_modify
	every</A> command, then the interval between 100 and
	1000 is divided as 900/18 = 50 steps, and it will generate the
	sequence of output timesteps:
	</P>
	<PRE>100,150,200,...950,1000,1500,2000,...9500,10000,15000,etc
	</PRE>
	<P>The stride(x,y,z) function uses the current timestep to generate a new
	timestep. X,y >= 0 and z > 0 and x <= y are required. The generated
	timesteps increase in increments of z, from x to y, i.e. it generates
	the sequece x,x+z,x+2z,...,y. If y-x is not a multiple of z, then
	similar to the way a for loop operates, the last value will be one
	that does not exceed y. For any current timestep, the next timestep
	in the sequence is returned. Thus if stride(1000,2000,100) is used
	in a variable by the <A HREF = "dump_modify.html">dump_modify every</A> command, it
	will generate the sequence of output timesteps:
	</P>
	<PRE>1000,1100,1200, ... ,1900,2000
	</PRE>
	<P>The stride2(x,y,z,a,b,c) function is similar to the stride() function
	except it generates two sets of strided timesteps, one at a coarser
	level and one at a finer level. Thus it is useful for debugging,
	e.g. to produce output every timestep at the point in simulation when
	a problem occurs. X,y >= 0 and z > 0 and x <= y are required, as are
	a,b >= 0 and c > 0 and a < b. Also, a >= x and b <= y are required so
	that the second stride is inside the first. The generated timesteps
	increase in increments of z, starting at x, until a is reached. At
	that point the timestep increases in increments of c, from a to b,
	then after b, increments by z are resumed until y is reached. For any
	current timestep, the next timestep in the sequence is returned. Thus
	if stride(1000,2000,100,1350,1360,1) is used in a variable by the
	<A HREF = "dump_modify.html">dump_modify every</A> command, it will generate the
	sequence of output timesteps:
	</P>
	<PRE>1000,1100,1200,1300,1350,1351,1352, ... 1359,1360,1400,1500, ... ,2000
	</PRE>
	<P>The vdisplace(x,y) function takes 2 arguments: x = value0 and y =
	velocity, and uses the elapsed time to change the value by a linear
	displacement due to the applied velocity over the course of a run,
	according to this formula:
	</P>
	<PRE>value = value0 + velocity(timestep-startstep)dt
	</PRE>
	<P>where dt = the timestep size.
	</P>
	<P>The run begins on startstep. Startstep can span multiple runs, using
	the <I>start</I> keyword of the <A HREF = "run.html">run</A> command. See the
	<A HREF = "run.html">run</A> command for details of how to do this. Note that the
	<A HREF = "thermo_style.html">thermo_style</A> keyword elaplong =
	timestep-startstep.
	</P>
	<P>The swiggle(x,y,z) and cwiggle(x,y,z) functions each take 3 arguments:
	x = value0, y = amplitude, z = period. They use the elapsed time to
	oscillate the value by a sin() or cos() function over the course of a
	run, according to one of these formulas, where omega = 2 PI / period:
	</P>
	<PRE>value = value0 + Amplitude * sin(omega(timestep-startstep)dt)
	value = value0 + Amplitude * (1 - cos(omega(timestep-startstep)dt))
	</PRE>
	<P>where dt = the timestep size.
	</P>
	<P>The run begins on startstep. Startstep can span multiple runs, using
	the <I>start</I> keyword of the <A HREF = "run.html">run</A> command. See the
	<A HREF = "run.html">run</A> command for details of how to do this. Note that the
	<A HREF = "thermo_style.html">thermo_style</A> keyword elaplong =
	timestep-startstep.
	</P>
	<HR>

	<H4>Group and Region Functions
	</H4>
	<P>Group functions are specified as keywords followed by one or two
	parenthesized arguments. The first argument <I>ID</I> is the group-ID.
	The <I>dim</I> argument, if it exists, is <I>x</I> or <I>y</I> or <I>z</I>. The <I>dir</I>
	argument, if it exists, is <I>xmin</I>, <I>xmax</I>, <I>ymin</I>, <I>ymax</I>, <I>zmin</I>, or
	<I>zmax</I>. The <I>dimdim</I> argument, if it exists, is <I>xx</I> or <I>yy</I> or <I>zz</I>
	or <I>xy</I> or <I>yz</I> or <I>xz</I>.
	</P>
	<P>The group function count() is the number of atoms in the group. The
	group functions mass() and charge() are the total mass and charge of
	the group. Xcm() and vcm() return components of the position and
	velocity of the center of mass of the group. Fcm() returns a
	component of the total force on the group of atoms. Bound() returns
	the min/max of a particular coordinate for all atoms in the group.
	Gyration() computes the radius-of-gyration of the group of atoms. See
	the <A HREF = "compute_gyration.html">compute gyration</A> command for a definition
	of the formula. Angmom() returns components of the angular momentum
	of the group of atoms around its center of mass. Torque() returns
	components of the torque on the group of atoms around its center of
	mass, based on current forces on the atoms. Inertia() returns one of
	6 components of the symmetric inertia tensor of the group of atoms
	around its center of mass, ordered as Ixx,Iyy,Izz,Ixy,Iyz,Ixz.
	Omega() returns components of the angular velocity of the group of
	atoms around its center of mass.
	</P>
	<P>Region functions are specified exactly the same way as group functions
	except they take an extra final argument <I>IDR</I> which is the region ID.
	The function is computed for all atoms that are in both the group and
	the region. If the group is "all", then the only criteria for atom
	inclusion is that it be in the region.
	</P>
	<HR>

	<H4>Special Functions
	</H4>
	<P>Special functions take specific kinds of arguments, meaning their
	arguments cannot be formulas themselves.
	</P>
	<P>The sum(x), min(x), max(x), ave(x), trap(x), and slope(x) functions
	each take 1 argument which is of the form "c_ID" or "c_ID[N]" or
	"f_ID" or "f_ID[N]". The first two are computes and the second two
	are fixes; the ID in the reference should be replaced by the ID of a
	compute or fix defined elsewhere in the input script. The compute or
	fix must produce either a global vector or array. If it produces a
	global vector, then the notation without "[N]" should be used. If
	it produces a global array, then the notation with "[N]" should be
	used, when N is an integer, to specify which column of the global
	array is being referenced.
	</P>
	<P>These functions operate on the global vector of inputs and reduce it
	to a single scalar value. This is analagous to the operation of the
	<A HREF = "compute_reduce.html">compute reduce</A> command, which invokes the same
	functions on per-atom and local vectors.
	</P>
	<P>The sum() function calculates the sum of all the vector elements. The
	min() and max() functions find the minimum and maximum element
	respectively. The ave() function is the same as sum() except that it
	divides the result by the length of the vector.
	</P>
	<P>The trap() function is the same as sum() except the first and last
	elements are multiplied by a weighting factor of 1/2 when performing
	the sum. This effectively implements an integration via the
	trapezoidal rule on the global vector of data. I.e. consider a set of
	points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2),
	..., (N,VN), where the Vi are the values in the global vector of
	length N. The integral from 1 to N of these points is trap(). When
	appropriately normalized by the timestep size, this function is useful
	for calculating integrals of time-series data, like that generated by
	the <A HREF = "fix_ave_correlate.html">fix ave/correlate</A> command.
	</P>
	<P>The slope() function uses linear regression to fit a line to the set
	of points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2),
	..., (N,VN), where the Vi are the values in the global vector of
	length N. The returned value is the slope of the line. If the line
	has a single point or is vertical, it returns 1.0e20.
	</P>
	<P>The gmask(x) function takes 1 argument which is a group ID. It
	can only be used in atom-style variables. It returns a 1 for
	atoms that are in the group, and a 0 for atoms that are not.
	</P>
	<P>The rmask(x) function takes 1 argument which is a region ID. It can
	only be used in atom-style variables. It returns a 1 for atoms that
	are in the geometric region, and a 0 for atoms that are not.
	</P>
	<P>The grmask(x,y) function takes 2 arguments. The first is a group ID,
	and the second is a region ID. It can only be used in atom-style
	variables. It returns a 1 for atoms that are in both the group and
	region, and a 0 for atoms that are not in both.
	</P>
	<P>The next(x) function takes 1 argument which is a variable ID (not
	"v_foo", just "foo"). It must be for a file-style or atomfile-style
	variable. Each time the next() function is invoked (i.e. each time
	the equal-style or atom-style variable is evaluated), the following
	steps occur.
	</P>
	<P>For file-style variables, the current string value stored by the
	file-style variable is converted to a numeric value and returned by
	the function. And the next string value in the file is read and
	stored. Note that if the line previously read from the file was not a
	numeric string, then it will typically evaluate to 0.0, which is
	likely not what you want.
	</P>
	<P>For atomfile-style variables, the current per-atom values stored by
	the atomfile-style variable are returned by the function. And the
	next set of per-atom values in the file is read and stored.
	</P>
	<P>Since file-style and atomfile-style variables read and store the first
	line of the file or first set of per-atoms values when they are
	defined in the input script, these are the value(s) that will be
	returned the first time the next() function is invoked. If next() is
	invoked more times than there are lines or sets of lines in the file,
	the variable is deleted, similar to how the <A HREF = "next.html">next</A> command
	operates.
	</P>
	<HR>

	<H4>Feature Functions
	</H4>
	<P>Feature functions allow to probe the running LAMMPS executable for
	whether specific features are either active, defined, or available.
	The functions take two arguments, a <I>category</I> and a corresponding
	<I>argument</I>. The arguments are strings thus cannot be formulas
	themselves (only $-style immediate variable expansion is possible).
	Return value is either 1.0 or 0.0 depending on whether the function
	evaluates to true or false, respectively.
	</P>
	<P>The <I>is_active()</I> function allows to query for active settings which
	are grouped by categories. Currently supported categories and
	arguments are:
	</P>
	<UL><LI><I>package</I> (argument = <I>cuda</I> or <I>gpu</I> or <I>intel</I> or <I>kokkos</I> or <I>omp</I>)
	<LI><I>newton</I> (argument = <I>pair</I> or <I>bond</I> or <I>any</I>)
	<LI><I>pair</I> (argument = <I>single</I> or <I>respa</I> or <I>manybody</I> or <I>tail</I> or <I>shift</I>)
	<LI><I>comm_style</I> (argument = <I>brick</I> or <I>tiled</I>)
	<LI><I>min_style</I> (argument = any of the compiled in minimizer styles)
	<LI><I>run_style</I> (argument = any of the compiled in run styles)
	<LI><I>atom_style</I> (argument = any of the compiled in atom styles)
	<LI><I>pair_style</I> (argument = any of the compiled in pair styles)
	<LI><I>bond_style</I> (argument = any of the compiled in bond styles)
	<LI><I>angle_style</I> (argument = any of the compiled in angle styles)
	<LI><I>dihedral_style</I> (argument = any of the compiled in dihedral styles)
	<LI><I>improper_style</I> (argument = any of the compiled in improper styles)
	<LI><I>kspace_style</I> (argument = any of the compiled in kspace styles)
	</UL>
	<P>Most of the settings are self-explanatory, the <I>single</I> argument in the
	<I>pair</I> category allows to check whether a pair style supports a
	Pair::single() function as needed by compute group/group and others
	features or LAMMPS, <I>respa</I> allows to check whether the inner/middle/outer
	mode of r-RESPA is supported. In the various style categories,
	the checking is also done using suffix flags, if available and enabled.
	</P>
	<P>Example 1: disable use of suffix for pppm when using GPU package (i.e. run it on the CPU concurrently to running the pair style on the GPU), but do use the suffix otherwise (e.g. with USER-OMP).
	</P>
	<PRE>pair_style lj/cut/coul/long 14.0
	if $(is_active(package,gpu)) then "suffix off"
	kspace_style pppm
	</PRE>
	<P>Example 2: use r-RESPA with inner/outer cutoff, if supported by pair style, otherwise fall back to using pair and reducing the outer time step
	</P>
	<PRE>timestep $(2.0(1.0+is_active(pair,respa))
	if $(is_active(pair,respa)) then "run_style respa 4 3 2 2 improper 1 inner 2 5.5 7.0 outer 3 kspace 4" else "run_style respa 3 3 2 improper 1 pair 2 kspace 3"
	</PRE>
	<P>The <I>is_defined()</I> function allows to query categories like <I>compute</I>,
	<I>dump</I>, <I>fix</I>, <I>group</I>, <I>region</I>, and <I>variable</I> whether an entry
	with the provided name or id is defined.
	</P>
	<P>The <I>is_available()</I> function allows to query whether a specific
	optional feature is available, i.e. compiled in. This currently
	works for the following categories: <I>command</I>, <I>compute</I>, <I>fix</I>,
	and <I>pair_style</I>. For all categories except <I>command</I> also appending
	active suffixes is tried before reporting failure.
	</P>
	<HR>

	<H4>Atom Values and Vectors
	</H4>
	<P>Atom values take an integer argument I from 1 to N, where I is the
	atom-ID, e.g. x[243], which means use the x coordinate of the atom
	with ID = 243. Or they can take a variable name, specified as v_name,
	where name is the name of the variable, like x[v_myIndex]. The
	variable can be of any style except atom or atom-file variables. The
	variable is evaluated and the result is expected to be numeric and is
	cast to an integer (i.e. 3.4 becomes 3), to use an an index, which
	must be a value from 1 to N. Note that a "formula" cannot be used as
	the argument between the brackets, e.g. x[243+10] or
	x[v_myIndex+1] are not allowed. To do this a single variable can be
	defined that contains the needed formula.
	</P>
	<P>Note that the 0 < atom-ID <= N, where N is the largest atom ID
	in the system. If an ID is specified for an atom that does not
	currently exist, then the generated value is 0.0.
	</P>
	<P>Atom vectors generate one value per atom, so that a reference like
	"vx" means the x-component of each atom's velocity will be used when
	evaluating the variable.
	</P>
	<P>The meaning of the different atom values and vectors is mostly
	self-explanatory. Mol refers to the molecule ID of an atom, and is
	only defined if an <A HREF = "atom_style.html">atom_style</A> is being used that
	defines molecule IDs.
	</P>
	<P>Note that many other atom attributes can be used as inputs to a
	variable by using the <A HREF = "compute_property_atom.html">compute
	property/atom</A> command and then specifying
	a quantity from that compute.
	</P>
	<HR>

	<H4>Compute References
	</H4>
	<P>Compute references access quantities calculated by a
	<A HREF = "compute.html">compute</A>. The ID in the reference should be replaced by
	the ID of a compute defined elsewhere in the input script. As
	discussed in the doc page for the <A HREF = "compute.html">compute</A> command,
	computes can produce global, per-atom, or local values. Only global
	and per-atom values can be used in a variable. Computes can also
	produce a scalar, vector, or array. An equal-style variable can only
	use scalar values, which means a global scalar, or an element of a
	global or per-atom vector or array. Atom-style variables can use the
	same scalar values. They can also use per-atom vector values. A
	vector value can be a per-atom vector itself, or a column of an
	per-atom array. See the doc pages for individual computes to see what
	kind of values they produce.
	</P>
	<P>Examples of different kinds of compute references are as follows.
	There is no ambiguity as to what a reference means, since computes
	only produce global or per-atom quantities, never both.
	</P>
	<DIV ALIGN=center><TABLE BORDER=1 >
	<TR><TD >c_ID</TD><TD > global scalar, or per-atom vector</TD></TR>
	<TR><TD >c_ID[I]</TD><TD > Ith element of global vector, or atom I's value in per-atom vector, or Ith column from per-atom array</TD></TR>
	<TR><TD >c_ID[I][J]</TD><TD > I,J element of global array, or atom I's Jth value in per-atom array
	</TD></TR></TABLE></DIV>

	<P>For I and J, integers can be specified or a variable name, specified
	as v_name, where name is the name of the variable. The rules for this
	syntax are the same as for the "Atom Values and Vectors" discussion
	above.
	</P>
	<P>If a variable containing a compute is evaluated directly in an input
	script (not during a run), then the values accessed by the compute
	must be current. See the discussion below about "Variable Accuracy".
	</P>
	<HR>

	<H4>Fix References
	</H4>
	<P>Fix references access quantities calculated by a <A HREF = "compute.html">fix</A>.
	The ID in the reference should be replaced by the ID of a fix defined
	elsewhere in the input script. As discussed in the doc page for the
	<A HREF = "fix.html">fix</A> command, fixes can produce global, per-atom, or local
	values. Only global and per-atom values can be used in a variable.
	Fixes can also produce a scalar, vector, or array. An equal-style
	variable can only use scalar values, which means a global scalar, or
	an element of a global or per-atom vector or array. Atom-style
	variables can use the same scalar values. They can also use per-atom
	vector values. A vector value can be a per-atom vector itself, or a
	column of an per-atom array. See the doc pages for individual fixes
	to see what kind of values they produce.
	</P>
	<P>The different kinds of fix references are exactly the same as the
	compute references listed in the above table, where "c_" is replaced
	by "f_". Again, there is no ambiguity as to what a reference means,
	since fixes only produce global or per-atom quantities, never both.
	</P>
	<DIV ALIGN=center><TABLE BORDER=1 >
	<TR><TD >f_ID</TD><TD > global scalar, or per-atom vector</TD></TR>
	<TR><TD >f_ID[I]</TD><TD > Ith element of global vector, or atom I's value in per-atom vector, or Ith column from per-atom array</TD></TR>
	<TR><TD >f_ID[I][J]</TD><TD > I,J element of global array, or atom I's Jth value in per-atom array
	</TD></TR></TABLE></DIV>

	<P>For I and J, integers can be specified or a variable name, specified
	as v_name, where name is the name of the variable. The rules for this
	syntax are the same as for the "Atom Values and Vectors" discussion
	above.
	</P>
	<P>If a variable containing a fix is evaluated directly in an input
	script (not during a run), then the values accessed by the fix should
	be current. See the discussion below about "Variable Accuracy".
	</P>
	<P>Note that some fixes only generate quantities on certain timesteps.
	If a variable attempts to access the fix on non-allowed timesteps, an
	error is generated. For example, the <A HREF = "fix_ave_time.html">fix ave/time</A>
	command may only generate averaged quantities every 100 steps. See
	the doc pages for individual fix commands for details.
	</P>
	<HR>

	<H4>Variable References
	</H4>
	<P>Variable references access quantities stored or calculated by other
	variables, which will cause those variables to be evaluated. The name
	in the reference should be replaced by the name of a variable defined
	elsewhere in the input script.
	</P>
	<P>As discussed on this doc page, equal-style variables generate a global
	scalar numeric value; atom-style and atomfile-style variables generate
	a per-atom vector of numeric values; all other variables store a
	string. The formula for an equal-style variable can use any style of
	variable except an atom-style or atomfile-style (unless only a single
	value from the variable is accessed via a subscript). If a
	string-storing variable is used, the string is converted to a numeric
	value. Note that this will typically produce a 0.0 if the string is
	not a numeric string, which is likely not what you want. The formula
	for an atom-style variable can use any style of variable, including
	other atom-style or atomfile-style variables.
	</P>
	<P>Examples of different kinds of variable references are as follows.
	There is no ambiguity as to what a reference means, since variables
	produce only a global scalar or a per-atom vector, never both.
	</P>
	<DIV ALIGN=center><TABLE BORDER=1 >
	<TR><TD >v_name</TD><TD > scalar, or per-atom vector</TD></TR>
	<TR><TD >v_name[I]</TD><TD > atom I's value in per-atom vector
	</TD></TR></TABLE></DIV>

	<P>For I, an integer can be specified or a variable name, specified as
	v_name, where name is the name of the variable. The rules for this
	syntax are the same as for the "Atom Values and Vectors" discussion
	above.
	</P>
	<HR>

	<P><B>Immediate Evaluation of Variables:</B>
	</P>
	<P>If you want an equal-style variable to be evaluated immediately, it
	may be the case that you do not need to define a variable at all. See
	<A HREF = "Section_commands.html#cmd_2">Section 3.2</A> of the manual, which
	describes the use of "immediate" variables in an input script,
	specified as $(formula) with parenthesis, where the formula has the
	same syntax as equal-style variables described on this page. This
	effectively evaluates a formula immediately without using the variable
	command to define a named variable.
	</P>
	<P>More generally, there is a difference between referencing a variable
	with a leading $ sign (e.g. $x or ${abc}) versus with a leading "v_"
	(e.g. v_x or v_abc). The former can be used in any input script
	command, including a variable command. The input script parser
	evaluates the reference variable immediately and substitutes its value
	into the command. As explained in <A HREF = "Section_commands.html#3_2">Section commands
	3.2</A> for "Parsing rules", you can also use
	un-named "immediate" variables for this purpose. For example, a
	string like this $((xlo+xhi)/2+sqrt(v_area)) in an input script
	command evaluates the string between the parenthesis as an equal-style
	variable formula.
	</P>
	<P>Referencing a variable with a leading "v_" is an optional or required
	kind of argument for some commands (e.g. the <A HREF = "fix_ave_spatial.html">fix
	ave/spatial</A> or <A HREF = "dump.html">dump custom</A> or
	<A HREF = "thermo_style.html">thermo_style</A> commands) if you wish it to evaluate
	a variable periodically during a run. It can also be used in a
	variable formula if you wish to reference a second variable. The
	second variable will be evaluated whenever the first variable is
	evaluated.
	</P>
	<P>As an example, suppose you use this command in your input script to
	define the variable "v" as
	</P>
	<PRE>variable v equal vol
	</PRE>
	<P>before a run where the simulation box size changes. You might think
	this will assign the initial volume to the variable "v". That is not
	the case. Rather it assigns a formula which evaluates the volume
	(using the thermo_style keyword "vol") to the variable "v". If you
	use the variable "v" in some other command like <A HREF = "fix_ave_time.html">fix
	ave/time</A> then the current volume of the box will be
	evaluated continuously during the run.
	</P>
	<P>If you want to store the initial volume of the system, you can do it
	this way:
	</P>
	<PRE>variable v equal vol
	variable v0 equal $v
	</PRE>
	<P>The second command will force "v" to be evaluated (yielding the
	initial volume) and assign that value to the variable "v0". Thus the
	command
	</P>
	<PRE>thermo_style custom step v_v v_v0
	</PRE>
	<P>would print out both the current and initial volume periodically
	during the run.
	</P>
	<P>Note that it is a mistake to enclose a variable formula in double
	quotes if it contains variables preceeded by $ signs. For example,
	</P>
	<PRE>variable vratio equal "${vfinal}/${v0}"
	</PRE>
	<P>This is because the quotes prevent variable substitution (see <A HREF = "Section_commands.html#cmd_2">this
	section</A> on parsing input script
	commands), and thus an error will occur when the formula for "vratio"
	is evaluated later.
	</P>
	<HR>

	<P><B>Variable Accuracy:</B>
	</P>
	<P>Obviously, LAMMPS attempts to evaluate variables containing formulas
	(<I>equal</I> and <I>atom</I> style variables) accurately whenever the
	evaluation is performed. Depending on what is included in the
	formula, this may require invoking a <A HREF = "compute.html">compute</A>, either
	directly or indirectly via a thermo keyword, or accessing a value
	previously calculated by a compute, or accessing a value calculated
	and stored by a <A HREF = "fix.html">fix</A>. If the compute is one that calculates
	the pressure or energy of the system, then these quantities need to be
	tallied during the evaluation of the interatomic potentials (pair,
	bond, etc) on timesteps that the variable will need the values.
	</P>
	<P>LAMMPS keeps track of all of this during a <A HREF = "run.html">run</A> or <A HREF = "minimize.html">energy
	minimization</A>. An error will be generated if you
	attempt to evaluate a variable on timesteps when it cannot produce
	accurate values. For example, if a <A HREF = "thermo_style.html">thermo_style
	custom</A> command prints a variable which accesses
	values stored by a <A HREF = "fix_ave_time.html">fix ave/time</A> command and the
	timesteps on which thermo output is generated are not multiples of the
	averaging frequency used in the fix command, then an error will occur.
	</P>
	<P>An input script can also request variables be evaluated before or
	after or in between runs, e.g. by including them in a
	<A HREF = "print.html">print</A> command. In this case, if a compute is needed to
	evaluate a variable (either directly or indirectly), LAMMPS will not
	invoke the compute, but it will use a value previously calculated by
	the compute, and can do this only if it was invoked on the current
	timestep. Fixes will always provide a quantity needed by a variable,
	but the quantity may or may not be current. This leads to one of
	three kinds of behavior:
	</P>
	<P>(1) The variable may be evaluated accurately. If it contains
	references to a compute or fix, and these values were calculated on
	the last timestep of a preceeding run, then they will be accessed and
	used by the variable and the result will be accurate.
	</P>
	<P>(2) LAMMPS may not be able to evaluate the variable and will generate
	an error message stating so. For example, if the variable requires a
	quantity from a <A HREF = "compute.html">compute</A> that has not been invoked on
	the current timestep, LAMMPS will generate an error. This means, for
	example, that such a variable cannot be evaluated before the first run
	has occurred. Likewise, in between runs, a variable containing a
	compute cannot be evaluated unless the compute was invoked on the last
	timestep of the preceding run, e.g. by thermodynamic output.
	</P>
	<P>One way to get around this problem is to perform a 0-timestep run
	before using the variable. For example, these commands
	</P>
	<PRE>variable t equal temp
	print "Initial temperature = $t"
	run 1000
	</PRE>
	<P>will generate an error if the run is the first run specified in the
	input script, because generating a value for the "t" variable requires
	a compute for calculating the temperature to be invoked.
	</P>
	<P>However, this sequence of commands would be fine:
	</P>
	<PRE>run 0
	variable t equal temp
	print "Initial temperature = $t"
	run 1000
	</PRE>
	<P>The 0-timestep run initializes and invokes various computes, including
	the one for temperature, so that the value it stores is current and
	can be accessed by the variable "t" after the run has completed. Note
	that a 0-timestep run does not alter the state of the system, so it
	does not change the input state for the 1000-timestep run that
	follows. Also note that the 0-timestep run must actually use and
	invoke the compute in question (e.g. via <A HREF = "thermo_style.html">thermo</A> or
	<A HREF = "dump.html">dump</A> output) in order for it to enable the compute to be
	used in a variable after the run. Thus if you are trying to print a
	variable that uses a compute you have defined, you can insure it is
	invoked on the last timestep of the preceding run by including it in
	thermodynamic output.
	</P>
	<P>Unlike computes, <A HREF = "fix.html">fixes</A> will never generate an error if
	their values are accessed by a variable in between runs. They always
	return some value to the variable. However, the value may not be what
	you expect if the fix has not yet calculated the quantity of interest
	or it is not current. For example, the <A HREF = "fix_indent.html">fix indent</A>
	command stores the force on the indenter. But this is not computed
	until a run is performed. Thus if a variable attempts to print this
	value before the first run, zeroes will be output. Again, performing
	a 0-timestep run before printing the variable has the desired effect.
	</P>
	<P>(3) The variable may be evaluated incorrectly and LAMMPS may have no
	way to detect this has occurred. Consider the following sequence of
	commands:
	</P>
	<PRE>pair_coeff 1 1 1.0 1.0
	run 1000
	pair_coeff 1 1 1.5 1.0
	variable e equal pe
	print "Final potential energy = $e"
	</PRE>
	<P>The first run is performed using one setting for the pairwise
	potential defined by the <A HREF = "pair_style.html">pair_style</A> and
	<A HREF = "pair_coeff.html">pair_coeff</A> commands. The potential energy is
	evaluated on the final timestep and stored by the <A HREF = "compute_pe.html">compute
	pe</A> compute (this is done by the
	<A HREF = "thermo_style.html">thermo_style</A> command). Then a pair coefficient is
	changed, altering the potential energy of the system. When the
	potential energy is printed via the "e" variable, LAMMPS will use the
	potential energy value stored by the <A HREF = "compute_pe.html">compute pe</A>
	compute, thinking it is current. There are many other commands which
	could alter the state of the system between runs, causing a variable
	to evaluate incorrectly.
	</P>
	<P>The solution to this issue is the same as for case (2) above, namely
	perform a 0-timestep run before the variable is evaluated to insure
	the system is up-to-date. For example, this sequence of commands
	would print a potential energy that reflected the changed pairwise
	coefficient:
	</P>
	<PRE>pair_coeff 1 1 1.0 1.0
	run 1000
	pair_coeff 1 1 1.5 1.0
	run 0
	variable e equal pe
	print "Final potential energy = $e"
	</PRE>
	<HR>

	<P><B>Restrictions:</B>
	</P>
	<P>Indexing any formula element by global atom ID, such as an atom value,
	requires the atom style to use a global mapping in order to look up
	the vector indices. By default, only atom styles with molecular
	information create global maps. The <A HREF = "atom_modify.html">atom_modify
	map</A> command can override the default.
	</P>
	<P>All <I>universe</I>- and <I>uloop</I>-style variables defined in an input script
	must have the same number of values.
	</P>
	<P><B>Related commands:</B>
	</P>
	<P><A HREF = "next.html">next</A>, <A HREF = "jump.html">jump</A>, <A HREF = "include.html">include</A>,
	<A HREF = "temper.html">temper</A>, <A HREF = "fix_print.html">fix print</A>, <A HREF = "print.html">print</A>
	</P>
	<P><B>Default:</B> none
	</P>
	</HTML>

variable.htmlNo OneTemporaryActions

File Metadata

variable.htmlView Options

Event Timeline

variable.html
No OneTemporary
Actions

variable.html
View Options