2015-07-30 22:38:28 +08:00
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<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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</CENTER>
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2006-09-22 00:22:34 +08:00
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2015-07-30 22:38:28 +08:00
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<HR>
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2006-09-22 00:22:34 +08:00
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2015-07-30 22:38:28 +08:00
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<H3>pair_coeff command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_coeff I J args
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</PRE>
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<UL><LI>I,J = atom types (see asterisk form below)
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<LI>args = coefficients for one or more pairs of atom types
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>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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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Specify the pairwise force field coefficients for one or more pairs of
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2006-09-22 00:22:34 +08:00
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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 <A HREF = "read_data.html">read_data</A> command or in a restart file.
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</P>
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<P>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 1st 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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</P>
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<P>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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</P>
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<P>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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</P>
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<PRE>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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</PRE>
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<P>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 1st example above would be listed
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as
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</P>
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<PRE>2 1.0 1.0 2.5
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</PRE>
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<P>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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<A HREF = "pair_modify.html">pair_modify</A> command for a discussion. Details on
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2007-06-25 22:36:36 +08:00
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this option as it pertains to individual potentials are described on
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the doc page for the potential.
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</P>
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<P>Many pair styles, typically for many-body potentials, use tabulated
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2013-06-29 01:09:43 +08:00
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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.
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</P>
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<P>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 the
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directory specified by the LAMMPS_POTENTIALS environment variable.
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Thus if this is set to the potentials directory in the LAMMPS distro,
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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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</P>
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<PRE>csh, tcsh:
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% setenv LAMMPS_POTENTIALS /path/to/lammps/potentials
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</PRE>
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<PRE>bash:
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% export LAMMPS_POTENTIALS=/path/to/lammps/potentials
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</PRE>
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<PRE>Windows:
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% set LAMMPS_POTENTIALS="C:\Path to LAMMPS\Potentials
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</PRE>
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<HR>
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<P>The alphabetic list of pair styles defined in LAMMPS is given on the
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<A HREF = "pair_style.html">pair_style</A> doc page. They are also given in more
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compact form in the pair section of <A HREF = "Section_commands.html#cmd_5">this
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page</A>.
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</P>
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<P>Click on the style to display the formula it computes, arguments
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specified in the pair_style command, and coefficients specified by the
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associated <A HREF = "pair_coeff.html">pair_coeff</A> command.
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</P>
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<P>Note that there are also additional pair styles (not listed on the
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<A HREF = "pair_style.html">pair_style</A> doc page) submitted by users which are
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included in the LAMMPS distribution. The list of these with links to
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the individual styles are given in the pair section of <A HREF = "Section_commands.html#cmd_5">this
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page</A>.
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</P>
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<P>There are also additional accelerated pair styles (not listed on the
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<A HREF = "pair_style.html">pair_style</A> doc page) included in the LAMMPS
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distribution for faster performance on CPUs and GPUs. The list of
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these with links to the individual styles are given in the pair
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section of <A HREF = "Section_commands.html#cmd_5">this page</A>.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This command must come after the simulation box is defined by a
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<A HREF = "read_data.html">read_data</A>, <A HREF = "read_restart.html">read_restart</A>, or
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<A HREF = "create_box.html">create_box</A> command.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_style.html">pair_style</A>, <A HREF = "pair_modify.html">pair_modify</A>,
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<A HREF = "read_data.html">read_data</A>, <A HREF = "read_restart.html">read_restart</A>,
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<A HREF = "pair_write.html">pair_write</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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