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