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<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>
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<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>granular</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 diameter</A> or <A HREF = "set.html">set
density</A> command.
</P>
<P>Also note that <A HREF = "pair_eam.html">pair_style eam</A> defines the masses of
atom types in the EAM potential file, 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. 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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