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mass.html
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Tue, Dec 3, 13:54
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rLAMMPS lammps
mass.html
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<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>
mass command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
mass I value
</PRE>
<UL><LI>
I = atom type (see asterisk form below)
<LI>
value = mass
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
mass 1 1.0
mass * 62.5
mass 2* 62.5
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Set the mass for all atoms of one or more atom types. Per-type mass
values can also be set in the
<A
HREF =
"read_data.html"
>
read_data
</A>
data file
using the "Masses" keyword. See the
<A
HREF =
"units.html"
>
units
</A>
command for
what mass units to use.
</P>
<P>
The I index can be specified in one of two ways. An explicit numeric
value can be used, as in the 1st example above. Or a wild-card
asterisk can be used to set the mass for multiple atom types. This
takes the form "*" or "*n" or "n*" or "m*n". If N = the number of
atom types, then an asterisk with no numeric values means all types
from 1 to N. A leading asterisk means all types from 1 to n
(inclusive). A trailing asterisk means all types from n to N
(inclusive). A middle asterisk means all types from m to n
(inclusive).
</P>
<P>
A line in a
<A
HREF =
"read_data.html"
>
data file
</A>
that follows the "Masses"
keyword specifies mass using the same format as the arguments of the
mass command in an input script, except that no wild-card asterisk can
be used. For example, under the "Masses" section of a data file, the
line that corresponds to the 1st example above would be listed as
</P>
<PRE>
1 1.0
</PRE>
<P>
Note that the mass command can only be used if the
<A
HREF =
"atom_style.html"
>
atom
style
</A>
requires per-type atom mass to be set.
Currently, all but the
<I>
sphere
</I>
and
<I>
ellipsoid
</I>
and
<I>
peri
</I>
styles do.
They require mass to be set for individual particles, not types.
Per-atom masses are defined in the data file read by the
<A
HREF =
"read_data.html"
>
read_data
</A>
command, or set to default values by the
<A
HREF =
"create_atoms.html"
>
create_atoms
</A>
command. Per-atom masses can also be
set to new values by the
<A
HREF =
"set.html"
>
set mass
</A>
or
<A
HREF =
"set.html"
>
set density
</A>
commands.
</P>
<P>
Also note that
<A
HREF =
"pair_eam.html"
>
pair_style eam
</A>
and
<A
HREF =
"pair_bop.html"
>
pair_style
bop
</A>
commands define the masses of atom types in their
respective potential files, in which case the mass command is normally
not used.
</P>
<P>
If you define a
<A
HREF =
"atom_style.html"
>
hybrid atom style
</A>
which includes one
(or more) sub-styles which require per-type mass and one (or more)
sub-styles which require per-atom mass, then you must define both.
However, in this case the per-type mass will be ignored; only the
per-atom mass will be used by LAMMPS.
</P>
<P><B>
Restrictions:
</B>
</P>
<P>
This command must come after the simulation box is defined by a
<A
HREF =
"read_data.html"
>
read_data
</A>
,
<A
HREF =
"read_restart.html"
>
read_restart
</A>
, or
<A
HREF =
"create_box.html"
>
create_box
</A>
command.
</P>
<P>
All masses must be defined before a simulation is run. They must also
all be defined before a
<A
HREF =
"velocity.html"
>
velocity
</A>
or
<A
HREF =
"fix_shake.html"
>
fix
shake
</A>
command is used.
</P>
<P>
The mass assigned to any type or atom must be > 0.0.
</P>
<P><B>
Related commands:
</B>
none
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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