mass command


mass I value
  • I = atom type (see asterisk form below), or type label

  • value = mass


mass 1 1.0
mass * 62.5
mass 2* 62.5

labelmap atom 1 C
mass C 12.01


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

The I index can be specified in one of several ways. An explicit numeric value can be used, as in the first example above. Or I can be a type label, which is an alphanumeric string defined by the labelmap command or in a section of a data file read by the read_data command, and which converts internally to a numeric type. 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”, where m and n are numbers. 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 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 first example above would be listed as

1 1.0

Note that the mass command can only be used if the 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 read_data command, or set to default values by the create_atoms command. Per-atom masses can also be set to new values by the set mass or set density commands.

Also note that pair_style eam and pair_style 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 hybrid 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.


This command must come after the simulation box is defined by a read_data, read_restart, or create_box command.

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

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