x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
c_ID = scalar or vector calculated by a compute with ID
c_ID[I] = Ith component of vector or Ith column of array calculated by a compute with ID, I can include wildcard (see below)
f_ID = scalar or vector calculated by a fix with ID
f_ID[I] = Ith component of vector or Ith column of array calculated by a fix with ID, I can include wildcard (see below)
v_name = value(s) calculated by an equal-style or vector-style or atom-style variable with name
v_name[I] = value calculated by a vector-style variable with name
</pre>
</li>
<li><pclass="first">zero or more keyword/arg pairs may be appended</p>
</li>
<li><pclass="first">keyword = <em>mode</em> or <em>file</em> or <em>ave</em> or <em>start</em> or <em>beyond</em> or <em>overwrite</em> or <em>title1</em> or <em>title2</em> or <em>title3</em></p>
<preclass="literal-block">
<em>mode</em> arg = <em>scalar</em> or <em>vector</em>
scalar = all input values are scalars
vector = all input values are vectors
<em>file</em> arg = filename
filename = name of file to output histogram(s) to
<em>ave</em> args = <em>one</em> or <em>running</em> or <em>window</em>
one = output a new average value every Nfreq steps
running = output cumulative average of all previous Nfreq steps
window M = output average of M most recent Nfreq steps
<em>start</em> args = Nstart
Nstart = start averaging on this timestep
<em>beyond</em> arg = <em>ignore</em> or <em>end</em> or <em>extra</em>
<p>Use one or more values as inputs every few timesteps to create a
single histogram. The histogram can then be averaged over longer
timescales. The resulting histogram can be used by other <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">output commands</span></a>, and can also be written to a
file. The fix ave/histo/weight command has identical syntax to fix
ave/histo, except that exactly two values must be specified. See
details below.</p>
<p>The group specified with this command is ignored for global and local
input values. For per-atom input values, only atoms in the group
contribute to the histogram. Note that regardless of the specified
group, specified values may represent calculations performed by
computes and fixes which store their own “group” definition.</p>
<p>A histogram is simply a count of the number of values that fall within
a histogram bin. <em>Nbins</em> are defined, with even spacing between <em>lo</em>
and <em>hi</em>. Values that fall outside the lo/hi bounds can be treated in
different ways; see the discussion of the <em>beyond</em> keyword below.</p>
<p>Each input value can be an atom attribute (position, velocity, force
component) or can be the result of a <aclass="reference internal"href="compute.html"><spanclass="doc">compute</span></a> or
<aclass="reference internal"href="fix.html"><spanclass="doc">fix</span></a> or the evaluation of an equal-style or vector-style or
atom-style <aclass="reference internal"href="variable.html"><spanclass="doc">variable</span></a>. The set of input values can be
either all global, all per-atom, or all local quantities. Inputs of
different kinds (e.g. global and per-atom) cannot be mixed. Atom
attributes are per-atom vector values. See the doc page for
individual “compute” and “fix” commands to see what kinds of
quantities they generate.</p>
<p>Note that the output of this command is a single histogram for all
input values combined together, not one histogram per input value.
See below for details on the format of the output of this fix.</p>
<p>The input values must either be all scalars or all vectors (or
arrays), depending on the setting of the <em>mode</em> keyword.</p>
<p>If <em>mode</em> = scalar, then the input values must be scalars, or vectors
with a bracketed term appended, indicating the Ith value of the vector
is used.</p>
<p>If <em>mode</em> = vector, then the input values must be vectors, or arrays
with a bracketed term appended, indicating the Ith column of the array
is used.</p>
<p>Note that for values from a compute or fix, the bracketed index I can
be specified using a wildcard asterisk with the index to effectively
specify multiple values. This takes the form “*” or “*n” or “n*” or
“m*n”. If N = the size of the vector (for <em>mode</em> = scalar) or the
number of columns in the array (for <em>mode</em> = vector), then an asterisk
with no numeric values means all indices from 1 to N. A leading
asterisk means all indices from 1 to n (inclusive). A trailing
asterisk means all indices from n to N (inclusive). A middle asterisk
means all indices from m to n (inclusive).</p>
<p>Using a wildcard is the same as if the individual elements of the
vector or columns of the array had been listed one by one. E.g. these
2 fix ave/histo commands are equivalent, since the <aclass="reference internal"href="compute_com_chunk.html"><spanclass="doc">compute com/chunk</span></a> command creates a global array with
<p>If the fix ave/histo/weight command is used, exactly two values must
be specified. If the values are vectors, they must be the same
length. The first value (a scalar or vector) is what is histogrammed
into bins, in the same manner the fix ave/histo command operates. The
second value (a scalar or vector) is used as a “weight”. This means
that instead of each value tallying a “1” to its bin, the
corresponding weight is tallied. E.g. The Nth entry (weight) in the
second vector is tallied to the bin corresponding to the Nth entry in
the first vector.</p>
<hrclass="docutils"/>
<p>The <em>Nevery</em>, <em>Nrepeat</em>, and <em>Nfreq</em> arguments specify on what
timesteps the input values will be used in order to contribute to the
histogram. The final histogram is generated on timesteps that are
multiple of <em>Nfreq</em>. It is averaged over <em>Nrepeat</em> histograms,
computed in the preceding portion of the simulation every <em>Nevery</em>
timesteps. <em>Nfreq</em> must be a multiple of <em>Nevery</em> and <em>Nevery</em> must
be non-zero even if <em>Nrepeat</em> is 1. Also, the timesteps
contributing to the histogram value cannot overlap,
i.e. Nrepeat*Nevery can not exceed Nfreq.</p>
<p>For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then input values
on timesteps 90,92,94,96,98,100 will be used to compute the final
histogram on timestep 100. Similarly for timesteps
190,192,194,196,198,200 on timestep 200, etc. If Nrepeat=1 and Nfreq
= 100, then no time averaging of the histogram is done; a histogram is
simply generated on timesteps 100,200,etc.</p>
<hrclass="docutils"/>
<p>The atom attribute values (x,y,z,vx,vy,vz,fx,fy,fz) are
self-explanatory. Note that other atom attributes can be used as
inputs to this fix by using the <aclass="reference internal"href="compute_property_atom.html"><spanclass="doc">compute property/atom</span></a> command and then specifying
an input value from that compute.</p>
<p>If a value begins with “c_”, a compute ID must follow which has been
previously defined in the input script. If <em>mode</em> = scalar, then if
no bracketed term is appended, the global scalar calculated by the
compute is used. If a bracketed term is appended, the Ith element of
the global vector calculated by the compute is used. If <em>mode</em> =
vector, then if no bracketed term is appended, the global or per-atom
or local vector calculated by the compute is used. If a bracketed
term is appended, the Ith column of the global or per-atom or local
array calculated by the compute is used. See the discussion above for
how I can be specified with a wildcard asterisk to effectively specify
multiple values.</p>
<p>Note that there is a <aclass="reference internal"href="compute_reduce.html"><spanclass="doc">compute reduce</span></a> command
which can sum per-atom quantities into a global scalar or vector which
can thus be accessed by fix ave/histo. Or it can be a compute defined
not in your input script, but by <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermodynamic output</span></a> or other fixes such as <aclass="reference internal"href="fix_nh.html"><spanclass="doc">fix nvt</span></a> or <aclass="reference internal"href="fix_temp_rescale.html"><spanclass="doc">fix temp/rescale</span></a>. See
the doc pages for these commands which give the IDs of these computes.
Users can also write code for their own compute styles and <aclass="reference internal"href="Section_modify.html"><spanclass="doc">add them to LAMMPS</span></a>.</p>
<p>If a value begins with “f_”, a fix ID must follow which has been
previously defined in the input script. If <em>mode</em> = scalar, then if
no bracketed term is appended, the global scalar calculated by the fix
is used. If a bracketed term is appended, the Ith element of the
global vector calculated by the fix is used. If <em>mode</em> = vector, then
if no bracketed term is appended, the global or per-atom or local
vector calculated by the fix is used. If a bracketed term is
appended, the Ith column of the global or per-atom or local array
calculated by the fix is used. See the discussion above for how I can
be specified with a wildcard asterisk to effectively specify multiple
values.</p>
<p>Note that some fixes only produce their values on certain timesteps,
which must be compatible with <em>Nevery</em>, else an error will result.
Users can also write code for their own fix styles and <aclass="reference internal"href="Section_modify.html"><spanclass="doc">add them to LAMMPS</span></a>.</p>
<p>If a value begins with “v_”, a variable name must follow which has
been previously defined in the input script. If <em>mode</em> = scalar, then
only equal-style or vector-style variables can be used, which both
produce global values. In this mode, a vector-style variable requires
a bracketed term to specify the Ith element of the vector calculated
by the variable. If <em>mode</em> = vector, then only vector-style or
atom-style variables can be used, which produce a global or per-atom
vector respectively. The vector-style variable must be used without a
bracketed term. See the <aclass="reference internal"href="variable.html"><spanclass="doc">variable</span></a> command for details.</p>
<p>Note that variables of style <em>equal</em>, <em>vector</em>, and <em>atom</em> define a
formula which can reference individual atom properties or
thermodynamic keywords, or they can invoke other computes, fixes, or
variables when they are evaluated, so this is a very general means of
specifying quantities to histogram.</p>
<hrclass="docutils"/>
<p>Additional optional keywords also affect the operation of this fix.</p>
<p>If the <em>mode</em> keyword is set to <em>scalar</em>, then all input values must
be global scalars, or elements of global vectors. If the <em>mode</em>
keyword is set to <em>vector</em>, then all input values must be global or
per-atom or local vectors, or columns of global or per-atom or local
arrays.</p>
<p>The <em>beyond</em> keyword determines how input values that fall outside the
<em>lo</em> to <em>hi</em> bounds are treated. Values such that <em>lo</em><= value <=
<em>hi</em> are assigned to one bin. Values on a bin boundary are assigned
to the lower of the 2 bins. If <em>beyond</em> is set to <em>ignore</em> then
values <<em>lo</em> and values ><em>hi</em> are ignored, i.e. they are not binned.
If <em>beyond</em> is set to <em>end</em> then values <<em>lo</em> are counted in the
first bin and values ><em>hi</em> are counted in the last bin. If <em>beyond</em>
is set to <em>extend</em> then two extra bins are created, so that there are
Nbins+2 total bins. Values <<em>lo</em> are counted in the first bin and
values ><em>hi</em> are counted in the last bin (Nbins+1). Values between
<em>lo</em> and <em>hi</em> (inclusive) are counted in bins 2 thru Nbins+1. The
“coordinate” stored and printed for these two extra bins is <em>lo</em> and
<em>hi</em>.</p>
<p>The <em>ave</em> keyword determines how the histogram produced every <em>Nfreq</em>
steps are averaged with histograms produced on previous steps that
were multiples of <em>Nfreq</em>, before they are accessed by another output
command or written to a file.</p>
<p>If the <em>ave</em> setting is <em>one</em>, then the histograms produced on
timesteps that are multiples of <em>Nfreq</em> are independent of each other;
they are output as-is without further averaging.</p>
<p>If the <em>ave</em> setting is <em>running</em>, then the histograms produced on
timesteps that are multiples of <em>Nfreq</em> are summed and averaged in a
cumulative sense before being output. Each bin value in the histogram
is thus the average of the bin value produced on that timestep with
all preceding values for the same bin. This running average begins
when the fix is defined; it can only be restarted by deleting the fix
via the <aclass="reference internal"href="unfix.html"><spanclass="doc">unfix</span></a> command, or by re-defining the fix by
re-specifying it.</p>
<p>If the <em>ave</em> setting is <em>window</em>, then the histograms produced on
timesteps that are multiples of <em>Nfreq</em> are summed within a moving
“window” of time, so that the last M histograms are used to produce
the output. E.g. if M = 3 and Nfreq = 1000, then the output on step
10000 will be the combined histogram of the individual histograms on
steps 8000,9000,10000. Outputs on early steps will be sums over less
than M histograms if they are not available.</p>
<p>The <em>start</em> keyword specifies what timestep histogramming will begin
on. The default is step 0. Often input values can be 0.0 at time 0,
so setting <em>start</em> to a larger value can avoid including a 0.0 in
a running or windowed histogram.</p>
<p>The <em>file</em> keyword allows a filename to be specified. Every <em>Nfreq</em>
steps, one histogram is written to the file. This includes a leading
line that contains the timestep, number of bins, the total count of
values contributing to the histogram, the count of values that were
not histogrammed (see the <em>beyond</em> keyword), the minimum value
encountered, and the maximum value encountered. The min/max values
include values that were not histogrammed. Following the leading
line, one line per bin is written into the file. Each line contains
the bin #, the coordinate for the center of the bin (between <em>lo</em> and
<em>hi</em>), the count of values in the bin, and the normalized count. The
normalized count is the bin count divided by the total count (not
including values not histogrammed), so that the normalized values sum
to 1.0 across all bins.</p>
<p>The <em>overwrite</em> keyword will continuously overwrite the output file
with the latest output, so that it only contains one timestep worth of
output. This option can only be used with the <em>ave running</em> setting.</p>
<p>The <em>title1</em> and <em>title2</em> and <em>title3</em> keywords allow specification of
the strings that will be printed as the first 3 lines of the output
file, assuming the <em>file</em> keyword was used. LAMMPS uses default
values for each of these, so they do not need to be specified.</p>
<p>By default, these header lines are as follows:</p>
<divclass="highlight-default"><divclass="highlight"><pre><span></span><spanclass="c1"># Histogram for fix ID</span>
<spanclass="c1"># Bin Coord Count Count/Total</span>
</pre></div>
</div>
<p>In the first line, ID is replaced with the fix-ID. The second line
describes the six values that are printed at the first of each section
of output. The third describes the 4 values printed for each bin in
the histogram.</p>
<hrclass="docutils"/>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>No information about this fix is written to <aclass="reference internal"href="restart.html"><spanclass="doc">binary restart files</span></a>. None of the <aclass="reference internal"href="fix_modify.html"><spanclass="doc">fix_modify</span></a> options
are relevant to this fix.</p>
<p>This fix produces a global vector and global array which can be
accessed by various <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">output commands</span></a>.
The values can only be accessed on timesteps that are multiples of
<em>Nfreq</em> since that is when a histogram is generated. The global
vector has 4 values:</p>
<ulclass="simple">
<li>1 = total counts in the histogram</li>
<li>2 = values that were not histogrammed (see <em>beyond</em> keyword)</li>
<li>3 = min value of all input values, including ones not histogrammed</li>
<li>4 = max value of all input values, including ones not histogrammed</li>
</ul>
<p>The global array has # of rows = Nbins and # of columns = 3. The
first column has the bin coordinate, the 2nd column has the count of
values in that histogram bin, and the 3rd column has the bin count
divided by the total count (not including missing counts), so that the
values in the 3rd column sum to 1.0.</p>
<p>The vector and array values calculated by this fix are all treated as
intensive. If this is not the case, e.g. due to histogramming
per-atom input values, then you will need to account for that when
interpreting the values produced by this fix.</p>
<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
the <aclass="reference internal"href="run.html"><spanclass="doc">run</span></a> command. This fix is not invoked during <aclass="reference internal"href="minimize.html"><spanclass="doc">energy minimization</span></a>.</p>
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