2006-11-04 01:09:44 +08:00
|
|
|
<HTML>
|
|
|
|
|
<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>
|
|
|
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<H3>pair_style sw command
|
|
|
|
|
</H3>
|
|
|
|
|
<P><B>Syntax:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>pair_style sw
|
|
|
|
|
</PRE>
|
|
|
|
|
<P><B>Examples:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>pair_style sw
|
|
|
|
|
pair_coeff * * si.sw Si
|
|
|
|
|
pair_coeff * * SiC.sw Si C Si
|
|
|
|
|
</PRE>
|
|
|
|
|
<P><B>Description:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P>The <I>sw</I> style computes a 3-body <A HREF = "#Stillinger">Stillinger-Weber</A>
|
|
|
|
|
potential for the energy E of a system of atoms as
|
|
|
|
|
</P>
|
|
|
|
|
<CENTER><IMG SRC = "Eqs/pair_sw.jpg">
|
|
|
|
|
</CENTER>
|
|
|
|
|
<P>where phi2 is a two-body term and phi3 is a three-body term. The
|
|
|
|
|
summations in the formula are over all neighbors J and K of atom I
|
|
|
|
|
within a cutoff distance = a*sigma.
|
|
|
|
|
</P>
|
|
|
|
|
<P>Only a single pair_coeff command is used with the <I>sw</I> style which
|
|
|
|
|
specifies a Stillinger-Weber potential file with parameters for all
|
|
|
|
|
needed elements. These are mapped to LAMMPS atom types by specifying
|
|
|
|
|
N additional arguments after the filename in the pair_coeff command,
|
|
|
|
|
where N is the number of LAMMPS atom types:
|
|
|
|
|
</P>
|
|
|
|
|
<UL><LI>filename
|
|
|
|
|
<LI>N element names = mapping of SW elements to atom types
|
|
|
|
|
</UL>
|
|
|
|
|
<P>As an example, imagine the SiC.sw file has Stillinger-Weber values for
|
|
|
|
|
Si and C. If your LAMMPS simulation has 4 atoms types and you want
|
|
|
|
|
the 1st 3 to be Si, and the 4th to be C, you would use the following
|
|
|
|
|
pair_coeff command:
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>pair_coeff * * SiC.sw Si Si Si C
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
|
|
|
|
|
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
|
|
|
|
|
element in the SW file. The final C argument maps LAMMPS atom type 4
|
|
|
|
|
to the C element in the SW file. If a mapping value is specified as
|
|
|
|
|
NULL, the mapping is not performed. This can be used when a <I>sw</I>
|
|
|
|
|
potential is used as part of the <I>hybrid</I> pair style. The NULL values
|
|
|
|
|
are placeholders for atom types that will be used with other
|
|
|
|
|
potentials.
|
|
|
|
|
</P>
|
|
|
|
|
<P>Stillinger-Weber files in the <I>potentials</I> directory of the LAMMPS
|
|
|
|
|
distribution have a ".sw" suffix. Lines that are not blank or
|
|
|
|
|
comments (starting with #) define parameters for a triplet of
|
2007-01-18 04:23:28 +08:00
|
|
|
elements. The parameters in a single entry correspond to the two-body
|
|
|
|
|
and three-body coefficients in the formula above:
|
2006-11-04 01:09:44 +08:00
|
|
|
</P>
|
|
|
|
|
<UL><LI>element 1 (the center atom in a 3-body interaction)
|
|
|
|
|
<LI>element 2
|
|
|
|
|
<LI>element 3
|
|
|
|
|
<LI>epsilon (energy units)
|
2007-01-18 07:59:33 +08:00
|
|
|
<LI>sigma (distance units)
|
2006-11-04 01:09:44 +08:00
|
|
|
<LI>a
|
|
|
|
|
<LI>lambda
|
|
|
|
|
<LI>gamma
|
|
|
|
|
<LI>costheta0
|
|
|
|
|
<LI>A
|
|
|
|
|
<LI>B
|
|
|
|
|
<LI>p
|
|
|
|
|
<LI>q
|
|
|
|
|
</UL>
|
2007-03-08 09:01:08 +08:00
|
|
|
<P>The A, B, p, and q parameters are used only for two-body
|
|
|
|
|
interactions. The lambda, gamma, and costheta0 parameters are used only for
|
|
|
|
|
three-body interactions. The epsilon, sigma and a parameters are used
|
|
|
|
|
for both two-body and three-body interactions.
|
|
|
|
|
The non-annotated parameters are unitless.
|
2006-11-04 01:09:44 +08:00
|
|
|
</P>
|
|
|
|
|
<P>The Stillinger-Weber potential file must contain entries for all the
|
|
|
|
|
elements listed in the pair_coeff command. It can also contain
|
|
|
|
|
entries for additional elements not being used in a particular
|
|
|
|
|
simulation; LAMMPS ignores those entries.
|
|
|
|
|
</P>
|
|
|
|
|
<P>For a single-element simulation, only a single entry is required
|
|
|
|
|
(e.g. SiSiSi). For a two-element simulation, the file must contain 8
|
|
|
|
|
entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
|
|
|
|
|
specify SW parameters for all permutations of the two elements
|
|
|
|
|
interacting in three-body configurations. Thus for 3 elements, 27
|
2007-01-18 04:23:28 +08:00
|
|
|
entries would be required, etc.
|
|
|
|
|
</P>
|
|
|
|
|
<P>As annotated above, the first element in the entry is the center atom
|
|
|
|
|
in a three-body interaction. Thus an entry for SiCC means a Si atom
|
|
|
|
|
with 2 C atoms as neighbors. By symmetry, three-body parameters for
|
2007-03-08 09:01:08 +08:00
|
|
|
SiCSi and SiSiC entries should be the same. The parameters used for
|
|
|
|
|
the two-body interaction come
|
|
|
|
|
from the entry where the 2nd element is repeated. Thus the two-body
|
|
|
|
|
parameters for Si interacting with C, comes from the SiCC entry.
|
|
|
|
|
Again by symmetry, the two-body parameters in the SiCC
|
|
|
|
|
and CSiSi entries should thus be the same.
|
|
|
|
|
The parameters used for a particular three-body
|
|
|
|
|
interaction come from the entry with the corresponding three elements.
|
|
|
|
|
The parameters used only for two-body interactions (A, B, p, and q)
|
|
|
|
|
in entries whose 2nd and 3rd element are different (e.g. SiCSi)
|
|
|
|
|
are not used for anything and can be set to 0.0 if desired.
|
2006-11-04 01:09:44 +08:00
|
|
|
</P>
|
2007-06-26 08:03:39 +08:00
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<P><B>Mixing, shift, table, tail correction, per-atom energy/stress, and
|
|
|
|
|
restart info</B>:
|
|
|
|
|
</P>
|
|
|
|
|
<P>For atom type pairs I,J and I != J, where types I and J correspond to
|
|
|
|
|
two different element types, mixing is performed by LAMMPS as
|
|
|
|
|
described above from values in the potential file.
|
|
|
|
|
</P>
|
|
|
|
|
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
|
|
|
|
|
shift, table, and tail options.
|
|
|
|
|
</P>
|
|
|
|
|
<P>This pair style does not calculate per-atom energy and stress, as used
|
|
|
|
|
by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
|
|
|
|
|
stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
|
|
|
|
|
commands.
|
|
|
|
|
</P>
|
|
|
|
|
<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
|
|
|
|
|
files</A>, since it is stored in potential files. Thus, you
|
|
|
|
|
need to re-specify the pair_style and pair_coeff commands in an input
|
|
|
|
|
script that reads a restart file.
|
|
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
2006-11-04 01:09:44 +08:00
|
|
|
<P><B>Restrictions:</B>
|
|
|
|
|
</P>
|
2007-06-26 08:03:39 +08:00
|
|
|
<P>This pair style is part of the "manybody" package. It is only enabled
|
|
|
|
|
if LAMMPS was built with that package (which it is by default). See
|
|
|
|
|
the <A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for more info.
|
|
|
|
|
</P>
|
|
|
|
|
<P>This pair style requires the <A HREF = "newton.html">newton</A> setting to be "on"
|
|
|
|
|
for pair interactions.
|
2006-11-04 01:09:44 +08:00
|
|
|
</P>
|
2007-04-24 00:55:05 +08:00
|
|
|
<P>The Stillinger-Weber potential files provided with LAMMPS (see the
|
|
|
|
|
potentials directory) are parameterized for metal <A HREF = "units.html">units</A>.
|
2007-06-26 08:03:39 +08:00
|
|
|
You can use the SW potential with any LAMMPS units, but you would need
|
|
|
|
|
to create your own SW potential file with coefficients listed in the
|
|
|
|
|
appropriate units if your simulation doesn't use "metal" units.
|
2007-04-24 00:55:05 +08:00
|
|
|
</P>
|
2006-11-04 01:09:44 +08:00
|
|
|
<P><B>Related commands:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P><A HREF = "pair_coeff.html">pair_coeff</A>
|
|
|
|
|
</P>
|
|
|
|
|
<P><B>Default:</B> none
|
|
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<A NAME = "Stillinger"></A>
|
|
|
|
|
|
|
|
|
|
<P><B>(Stillinger)</B> Stillinger and Weber, Phys Rev B, 31, 5262 (1985).
|
|
|
|
|
</P>
|
|
|
|
|
</HTML>
|
