lammps/doc/mass.txt

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:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

mass command :h3

[Syntax:]

mass I value :pre

I = atom type (see asterisk form below)
value = mass :ul

[Examples:]

mass 1 1.0
mass * 62.5
mass 2* 62.5 :pre

[Description:]

Set the mass for all atoms of one or more atom types.  Per-type mass
values can also be set in the "read_data"_read_data.html data file
using the "Masses" keyword.  See the "units"_units.html command for
what mass units to use.

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).

A line in a "data file"_read_data.html 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

1 1.0 :pre

Note that the mass command can only be used if the "atom
style"_atom_style.html requires per-type atom mass to be set.
Currently, all but the {sphere} and {ellipsoid} and {peri} 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
"read_data"_read_data.html command, or set to default values by the
"create_atoms"_create_atoms.html command.  Per-atom masses can also be
set to new values by the "set mass"_set.html or "set density"_set.html
commands.

Also note that "pair_style eam"_pair_eam.html defines the masses of
atom types in the EAM potential file, in which case the mass command
is normally not used.

If you define a "hybrid atom style"_atom_style.html 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.

[Restrictions:]

This command must come after the simulation box is defined by a
"read_data"_read_data.html, "read_restart"_read_restart.html, or
"create_box"_create_box.html command.

All masses must be defined before a simulation is run.  They must also
all be defined before a "velocity"_velocity.html or "fix
shake"_fix_shake.html command is used.

The mass assigned to any type or atom must be > 0.0.

[Related commands:] none

[Default:] none