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.. index:: mass
mass command
============
Syntax
""""""
.. parsed-literal::
mass I value
* I = atom type (see asterisk form below)
* value = mass
Examples
""""""""
.. parsed-literal::
mass 1 1.0
mass * 62.5
mass 2* 62.5
Description
"""""""""""
Set the mass for all atoms of one or more atom types. Per-type mass
values can also be set in the :doc:`read_data <read_data>` data file
using the "Masses" keyword. See the :doc:`units <units>` 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 :doc:`data file <read_data>` 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
.. parsed-literal::
1 1.0
Note that the mass command can only be used if the :doc:`atom style <atom_style>` 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
:doc:`read_data <read_data>` command, or set to default values by the
:doc:`create_atoms <create_atoms>` command. Per-atom masses can also be
set to new values by the :doc:`set mass <set>` or :doc:`set density <set>`
commands.
Also note that :doc:`pair_style eam <pair_eam>` and :doc:`pair_style bop <pair_bop>` commands define the masses of atom types in their
respective potential files, in which case the mass command is normally
not used.
If you define a :doc:`hybrid atom style <atom_style>` 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
:doc:`read_data <read_data>`, :doc:`read_restart <read_restart>`, or
:doc:`create_box <create_box>` command.
All masses must be defined before a simulation is run. They must also
all be defined before a :doc:`velocity <velocity>` or :doc:`fix shake <fix_shake>` command is used.
The mass assigned to any type or atom must be > 0.0.
**Related commands:** none
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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