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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<HR>
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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 * * GaN.sw Ga N Ga
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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 a file SiC.sw 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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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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</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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<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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<LI>tol
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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 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. gamma is used only in the
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three-body interactions, but is defined for pairs of atoms.
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The non-annotated parameters are unitless.
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</P>
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<P>LAMMPS introduces an additional performance-optimization parameter tol
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that is used for both two-body and three-body interactions. In the
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Stillinger-Weber potential, the interaction energies become negligibly
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small at atomic separations substantially less than the theoretical
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cutoff distances. LAMMPS therefore defines a virtual cutoff distance
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based on a user defined tolerance tol. The use of the virtual cutoff
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distance in constructing atom neighbor lists can significantly reduce
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the neighbor list sizes and therefore the computational cost. LAMMPS
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provides a <I>tol</I> value for each of the three-body entries so that they
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can be separately controlled. If tol = 0.0, then the standard
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Stillinger-Weber cutoff is used.
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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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2009-08-11 04:13:44 +08:00
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with 2 C atoms as neighbors. The parameter values used for the
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two-body interaction come from the entry where the 2nd and 3rd
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elements are the same. Thus the two-body parameters for Si
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interacting with C, comes from the SiCC entry. The three-body
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parameters can in principle be specific to the three elements of the
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configuration. In the literature, however, the three-body parameters
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are usually defined by simple formulas involving two sets of pair-wise
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parameters, corresponding to the ij and ik pairs, where i is the
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center atom. The user must ensure that the correct combining rule is
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used to calculate the values of the threebody parameters for
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alloys. Note also that the function phi3 contains two exponential
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screening factors with parameter values from the ij pair and ik
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pairs. So phi3 for a C atom bonded to a Si atom and a second C atom
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will depend on the three-body parameters for the CSiC entry, and also
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on the two-body parameters for the CCC and CSiSi entries. Since the
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order of the two neighbors is arbitrary, the threebody parameters for
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entries CSiC and CCSi should be the same. Similarly, the two-body
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parameters for entries SiCC and CSiSi should also be the same. The
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parameters used only for two-body interactions (A, B, p, and q) in
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entries whose 2nd and 3rd element are different (e.g. SiCSi) are not
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used for anything and can be set to 0.0 if desired.
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</P>
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<HR>
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2008-01-22 02:12:34 +08:00
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, where types I and J correspond to
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two different element types, mixing is performed by LAMMPS as
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described above from values in the potential file.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift, table, and tail options.
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</P>
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<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
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files</A>, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.
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</P>
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2007-10-04 08:21:14 +08:00
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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2006-11-04 01:09:44 +08:00
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<P><B>Restrictions:</B>
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</P>
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<P>This pair style is part of the "manybody" package. It is only enabled
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if LAMMPS was built with that package (which it is by default). See
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the <A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for more info.
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</P>
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<P>This pair style requires the <A HREF = "newton.html">newton</A> setting to be "on"
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for pair interactions.
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</P>
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2007-04-24 00:55:05 +08:00
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<P>The Stillinger-Weber potential files provided with LAMMPS (see the
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potentials directory) are parameterized for metal <A HREF = "units.html">units</A>.
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You can use the SW potential with any LAMMPS units, but you would need
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to create your own SW potential file with coefficients listed in the
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appropriate units if your simulation doesn't use "metal" units.
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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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