101 lines
3.2 KiB
ReStructuredText
101 lines
3.2 KiB
ReStructuredText
.. index:: mass
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mass command
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============
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Syntax
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""""""
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.. code-block:: LAMMPS
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mass I value
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* I = atom type (see asterisk form below), or type label
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* value = mass
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Examples
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""""""""
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.. code-block:: LAMMPS
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mass 1 1.0
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mass * 62.5
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mass 2* 62.5
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labelmap atom 1 C
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mass C 12.01
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Description
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"""""""""""
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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 :doc:`read_data <read_data>` data file
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using the "Masses" keyword. See the :doc:`units <units>` command for
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what mass units to use.
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The I index can be specified in one of several ways. An explicit
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numeric value can be used, as in the first example above. Or I can be
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a type label, which is an alphanumeric string defined by the
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:doc:`labelmap <labelmap>` command or in a section of a data file read
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by the :doc:`read_data <read_data>` command, and which converts
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internally to a numeric type. Or a wild-card asterisk can be used to
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set the mass for multiple atom types. This takes the form "\*" or
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"\*n" or "n\*" or "m\*n", where m and n are numbers. If N = the
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number of atom types, then an asterisk with no numeric values means
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all types 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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A line in a :doc:`data file <read_data>` 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 first example above would be listed as
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.. parsed-literal::
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1 1.0
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Note that the mass command can only be used if the :doc:`atom style <atom_style>` requires per-type atom mass to be set.
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Currently, all but the *sphere* and *ellipsoid* and *peri* styles do.
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They require mass to be set for individual particles, not types.
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Per-atom masses are defined in the data file read by the
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:doc:`read_data <read_data>` command, or set to default values by the
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:doc:`create_atoms <create_atoms>` command. Per-atom masses can also be
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set to new values by the :doc:`set mass <set>` or :doc:`set density <set>`
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commands.
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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
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respective potential files, in which case the mass command is normally
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not used.
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If you define a :doc:`hybrid atom style <atom_style>` 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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Restrictions
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""""""""""""
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This command must come after the simulation box is defined by a
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:doc:`read_data <read_data>`, :doc:`read_restart <read_restart>`, or
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:doc:`create_box <create_box>` command.
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All masses must be defined before a simulation is run. They must also
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all be defined before a :doc:`velocity <velocity>` or :doc:`fix shake <fix_shake>` command is used.
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The mass assigned to any type or atom must be > 0.0.
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Related commands
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""""""""""""""""
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none
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Default
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"""""""
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none
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