173 lines
6.4 KiB
ReStructuredText
173 lines
6.4 KiB
ReStructuredText
.. index:: pair_coeff
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pair_coeff 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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pair_coeff I J args
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* I,J = atom types (see asterisk form below)
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* args = coefficients for one or more pairs of atom types
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Examples
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""""""""
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.. code-block:: LAMMPS
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pair_coeff 1 2 1.0 1.0 2.5
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pair_coeff 2 * 1.0 1.0
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pair_coeff 3* 1*2 1.0 1.0 2.5
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pair_coeff * * 1.0 1.0
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pair_coeff * * nialhjea 1 1 2
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pair_coeff * 3 morse.table ENTRY1
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pair_coeff 1 2 lj/cut 1.0 1.0 2.5 (for pair_style hybrid)
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Description
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"""""""""""
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Specify the pairwise force field coefficients for one or more pairs of
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atom types. The number and meaning of the coefficients depends on the
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pair style. Pair coefficients can also be set in the data file read
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by the :doc:`read_data <read_data>` command or in a restart file.
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I and J can be specified in one of two ways. Explicit numeric values
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can be used for each, as in the first example above. I <= J is
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required. LAMMPS sets the coefficients for the symmetric J,I
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interaction to the same values.
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A wildcard asterisk can be used in place of or in conjunction with the
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I,J arguments to set the coefficients for multiple pairs of atom
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types. This takes the form "\*" or "\*n" or "n\*" or "m\*n". If N = the
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number of atom types, then an asterisk with no numeric values means all
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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). Note that only type pairs with I <= J are considered; if
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asterisks imply type pairs where J < I, they are ignored.
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Note that a pair_coeff command can override a previous setting for the
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same I,J pair. For example, these commands set the coeffs for all I,J
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pairs, then overwrite the coeffs for just the I,J = 2,3 pair:
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.. code-block:: LAMMPS
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pair_coeff * * 1.0 1.0 2.5
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pair_coeff 2 3 2.0 1.0 1.12
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A line in a data file that specifies pair coefficients uses the exact
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same format as the arguments of the pair_coeff command in an input
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script, with the exception of the I,J type arguments. In each line of
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the "Pair Coeffs" section of a data file, only a single type I is
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specified, which sets the coefficients for type I interacting with
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type I. This is because the section has exactly N lines, where N =
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the number of atom types. For this reason, the wild-card asterisk
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should also not be used as part of the I argument. Thus in a data
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file, the line corresponding to the first example above would be listed
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as
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.. parsed-literal::
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2 1.0 1.0 2.5
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For many potentials, if coefficients for type pairs with I != J are
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not set explicitly by a pair_coeff command, the values are inferred
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from the I,I and J,J settings by mixing rules; see the
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:doc:`pair_modify <pair_modify>` command for a discussion. Details on
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this option as it pertains to individual potentials are described on
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the page for the potential.
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Many pair styles, typically for many-body potentials, use tabulated
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potential files as input, when specifying the pair_coeff command.
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Potential files provided with LAMMPS are in the potentials directory
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of the distribution. For some potentials, such as EAM, other archives
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of suitable files can be found on the Web. They can be used with
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LAMMPS so long as they are in the format LAMMPS expects, as discussed
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on the individual doc pages. The first line of potential files may
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contain metadata with upper case tags followed their value. These may
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be parsed and used by LAMMPS. Currently supported are the "DATE:"
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tag and the ``UNITS:`` tag. For pair styles that have been programmed
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to support the metadata, the value of the "DATE:" tag is printed to
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the screen and logfile so that the version of a potential file can be
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later identified. The ``UNITS:`` tag indicates the :doc:`units <units>`
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setting required for this particular potential file. If the potential
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file was created for a different sets of units, LAMMPS will terminate
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with an error. If the potential file does not contain the tag, no
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check will be made and it is the responsibility of the user to determine
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that the unit style is correct.
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In some select cases and for specific combinations of unit styles,
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LAMMPS is capable of automatically converting potential parameters
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from a file. In those cases, a warning message signaling that an
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automatic conversion has happened is printed to the screen.
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When a pair_coeff command using a potential file is specified, LAMMPS
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looks for the potential file in 2 places. First it looks in the
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location specified. E.g. if the file is specified as "niu3.eam", it
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is looked for in the current working directory. If it is specified as
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"../potentials/niu3.eam", then it is looked for in the potentials
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directory, assuming it is a sister directory of the current working
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directory. If the file is not found, it is then looked for in one of
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the directories specified by the ``LAMMPS_POTENTIALS`` environment variable.
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Thus if this is set to the potentials directory in the LAMMPS distribution,
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then you can use those files from anywhere on your system, without
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copying them into your working directory. Environment variables are
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set in different ways for different shells. Here are example settings
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for
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csh, tcsh:
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.. parsed-literal::
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% setenv LAMMPS_POTENTIALS /path/to/lammps/potentials
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bash:
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.. parsed-literal::
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% export LAMMPS_POTENTIALS=/path/to/lammps/potentials
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Windows:
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.. parsed-literal::
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% set LAMMPS_POTENTIALS="C:\\Path to LAMMPS\\Potentials"
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The ``LAMMPS_POTENTIALS`` environment variable may contain paths
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to multiple folders, if they are separated by ";" on Windows and
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":" on all other operating systems, just like the ``PATH`` and
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similar environment variables.
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----------
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The alphabetic list of pair styles defined in LAMMPS is given on the
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:doc:`pair_style <pair_style>` doc page. They are also listed in more
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compact form on the :doc:`Commands pair <Commands_pair>` doc page.
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Click on the style to display the formula it computes and its
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coefficients as specified by the associated pair_coeff command.
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----------
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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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Related commands
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""""""""""""""""
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:doc:`pair_style <pair_style>`, :doc:`pair_modify <pair_modify>`,
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:doc:`read_data <read_data>`, :doc:`read_restart <read_restart>`,
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:doc:`pair_write <pair_write>`
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Default
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"""""""
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none
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