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"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
: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
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