2006-11-04 01:09:44 +08:00
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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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<H3>pair_style sw 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_style sw
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style sw
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pair_coeff * * si.sw Si
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pair_coeff * * SiC.sw Si C Si
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>sw</I> style computes a 3-body <A HREF = "#Stillinger">Stillinger-Weber</A>
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potential for the energy E of a system of atoms as
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</P>
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<CENTER><IMG SRC = "Eqs/pair_sw.jpg">
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</CENTER>
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<P>where phi2 is a two-body term and phi3 is a three-body term. The
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summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = a*sigma.
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</P>
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<P>Only a single pair_coeff command is used with the <I>sw</I> style which
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specifies a Stillinger-Weber potential file with parameters for all
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needed elements. These are mapped to LAMMPS atom types by specifying
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N additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:
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</P>
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<UL><LI>filename
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<LI>N element names = mapping of SW elements to atom types
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</UL>
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<P>As an example, imagine the SiC.sw file has Stillinger-Weber values for
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Si and C. If your LAMMPS simulation has 4 atoms types and you want
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the 1st 3 to be Si, and the 4th to be C, you would use the following
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pair_coeff command:
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</P>
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<PRE>pair_coeff * * SiC.sw Si Si Si C
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</PRE>
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<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
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element in the SW file. The final C argument maps LAMMPS atom type 4
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to the C element in the SW file. If a mapping value is specified as
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NULL, the mapping is not performed. This can be used when a <I>sw</I>
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potential is used as part of the <I>hybrid</I> pair style. The NULL values
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are placeholders for atom types that will be used with other
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potentials.
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</P>
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<P>Stillinger-Weber files in the <I>potentials</I> directory of the LAMMPS
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distribution have a ".sw" suffix. Lines that are not blank or
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comments (starting with #) define parameters for a triplet of
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2007-01-18 04:23:28 +08:00
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elements. The parameters in a single entry correspond to the two-body
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and three-body coefficients in the formula above:
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2006-11-04 01:09:44 +08:00
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</P>
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<UL><LI>element 1 (the center atom in a 3-body interaction)
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<LI>element 2
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<LI>element 3
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<LI>epsilon (energy units)
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<LI>sigma (distance units)
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2006-11-04 01:09:44 +08:00
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<LI>a
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<LI>lambda
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<LI>gamma
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<LI>costheta0
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<LI>A
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<LI>B
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<LI>p
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<LI>q
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</UL>
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<P>The A, B, p, and q parameters are used only for two-body
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interactions. The lambda, gamma, and costheta0 parameters are used only for
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three-body interactions. The epsilon, sigma and a parameters are used
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for both two-body and three-body interactions.
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The non-annotated parameters are unitless.
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2006-11-04 01:09:44 +08:00
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</P>
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<P>The Stillinger-Weber potential file must contain entries for all the
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elements listed in the pair_coeff command. It can also contain
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entries for additional elements not being used in a particular
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simulation; LAMMPS ignores those entries.
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</P>
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<P>For a single-element simulation, only a single entry is required
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(e.g. SiSiSi). For a two-element simulation, the file must contain 8
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entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
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specify SW parameters for all permutations of the two elements
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interacting in three-body configurations. Thus for 3 elements, 27
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entries would be required, etc.
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</P>
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<P>As annotated above, the first element in the entry is the center atom
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in a three-body interaction. Thus an entry for SiCC means a Si atom
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with 2 C atoms as neighbors. By symmetry, three-body parameters for
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SiCSi and SiSiC entries should be the same. The parameters used for
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the two-body interaction come
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from the entry where the 2nd element is repeated. Thus the two-body
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parameters for Si interacting with C, comes from the SiCC entry.
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Again by symmetry, the two-body parameters in the SiCC
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and CSiSi entries should thus be the same.
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The parameters used for a particular three-body
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interaction come from the entry with the corresponding three elements.
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The parameters used only for two-body interactions (A, B, p, and q)
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in entries whose 2nd and 3rd element are different (e.g. SiCSi)
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are not used for anything and can be set to 0.0 if desired.
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2006-11-04 01:09:44 +08:00
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This pair potential requires the <A HREF = "newton.html">newton</A> setting to be
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"on" for pair interactions.
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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_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Stillinger"></A>
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<P><B>(Stillinger)</B> Stillinger and Weber, Phys Rev B, 31, 5262 (1985).
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</P>
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</HTML>
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