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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>
</CENTER>
<HR>
<H3>pair_style vashishta command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style vashishta
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style vashishta
pair_coeff * * SiC.vashishta Si C
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>vashishta</I> style computes the combined 2-body and 3-body
family of potentials developed in the group of Vashishta and
co-workers. By combining repulsive, screened Coulombic,
screened charge-dipole, and dispersion interactions with a
bond-angle energy based on the Stillinger-Weber potential,
this potential has been used to describe a variety of inorganic
compounds, including SiO2 <A HREF = "#Vashishta1990">Vashishta1990</A>,
SiC <A HREF = "#Vashishta2007">Vashishta2007</A>,
and InP <A HREF = "#Branicio2009">Branicio2009</A>.
</P>
<P>The potential for the energy U of a system of atoms is
</P>
<CENTER><IMG SRC = "Eqs/pair_vashishta.jpg">
</CENTER>
<P>where we follow the notation used in <A HREF = "#Branicio2009">Branicio2009</A>.
U2 is a two-body term and U3 is a three-body term. The
summation over two-body terms is over all neighbors J within
a cutoff distance = <I>rc</I>. The twobody terms are shifted and
tilted by a linear function so that the energy and force are
both zero at <I>rc</I>. The summation over three-body terms
is over all neighbors J and K within a cut-off distance = <I>r0</I>,
where the exponential screening function becomes zero.
</P>
<P>Only a single pair_coeff command is used with the <I>vashishta</I> style which
specifies a Vashishta 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 Vashishta elements to atom types
</UL>
<P>See the <A HREF = "pair_coeff.html">pair_coeff</A> doc page for alternate ways
to specify the path for the potential file.
</P>
<P>As an example, imagine a file SiC.vashishta has parameters 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.vashishta 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 file. The final C argument maps LAMMPS atom type 4
to the C element in the file. If a mapping value is specified as
NULL, the mapping is not performed. This can be used when a <I>vashishta</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>Vashishta files in the <I>potentials</I> directory of the LAMMPS
distribution have a ".vashishta" suffix. Lines that are not blank or
comments (starting with #) define parameters for a triplet of
elements. The parameters in a single entry correspond to the two-body
and three-body coefficients in the formulae above:
</P>
<UL><LI>element 1 (the center atom in a 3-body interaction)
<LI>element 2
<LI>element 3
<LI>H (energy units)
<LI>eta
<LI>Zi (electron charge units)
<LI>Zj (electron charge units)
<LI>lambda1 (distance units)
<LI>D (energy units)
<LI>lambda4 (distance units)
<LI>W (energy units)
<LI>rc (distance units)
<LI>B (energy units)
<LI>gamma
<LI>r0 (distance units)
<LI>C
<LI>costheta0
</UL>
<P>The non-annotated parameters are unitless.
The Vashishta 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.
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 parameters for all permutations of the two elements
interacting in three-body configurations. Thus for 3 elements, 27
entries would be required, etc.
</P>
<P>Depending on the particular version of the Vashishta potential,
the values of these parameters may be keyed to the identities of
zero, one, two, or three elements.
In order to make the input file format unambiguous, general,
and simple to code,
LAMMPS uses a slightly confusing method for specifying parameters.
All parameters are divided into two classes: two-body and three-body.
Two-body and three-body parameters are handled differently,
as described below.
The two-body parameters are H, eta, lambda1, D, lambda4, W, rc, gamma, and r0.
They appear in the above formulae with two subscripts.
The parameters Zi and Zj are also classified as two-body parameters,
even though they only have 1 subscript.
The three-body parameters are B, C, costheta0.
They appear in the above formulae with three subscripts.
Two-body and three-body parameters are handled differently,
as described below.
</P>
<P>The first element in each entry is the center atom
in a three-body interaction, while the second and third elements
are two neighbor atoms. Three-body parameters for a central atom I
and two neighbors J and K are taken from the IJK entry.
Note that even though three-body parameters do not depend on the order of
J and K, LAMMPS stores three-body parameters for both IJK and IKJ.
The user must ensure that these values are equal.
Two-body parameters for an atom I interacting with atom J are taken from
the IJJ entry, where the 2nd and 3rd
elements are the same. Thus the two-body parameters
for Si interacting with C come from the SiCC entry. Note that even
though two-body parameters (except possibly gamma and r0 in U3)
do not depend on the order of the two elements,
LAMMPS will get the Si-C value from the SiCC entry
and the C-Si value from the CSiSi entry. The user must ensure
that these values are equal. Two-body parameters appearing
in entries where the 2nd and 3rd elements are different are
stored but never used. It is good practice to enter zero for
these values. Note that the three-body function U3 above
contains the two-body parameters gamma and r0. So U3 for a
central C atom bonded to an Si atom and a second C atom
will take three-body parameters from the CSiC entry, but
two-body parameters from the CCC and CSiSi entries.
</P>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA 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 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>
<P>This pair style can only be used via the <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command. It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<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#start_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.
</P>
<P>The Vashishta potential files provided with LAMMPS (see the
potentials directory) are parameterized for metal <A HREF = "units.html">units</A>.
You can use the Vashishta potential with any LAMMPS units, but you would need
to create your own Vashishta potential file with coefficients listed in the
appropriate units if your simulation doesn't use "metal" units.
</P>
<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 = "Vashishta1990"></A>
<P><B>(Vashishta1990)</B> P. Vashishta, R. K. Kalia, J. P. Rino, Phys. Rev. B 41, 12197 (1990).
</P>
<A NAME = "Vashishta2007"></A>
<P><B>(Vashishta2007)</B> P. Vashishta, R. K. Kalia, A. Nakano, J. P. Rino. J. Appl. Phys. 101, 103515 (2007).
</P>
<A NAME = "Branicio2009"></A>
<P><B>(Branicio2009)</B> Branicio, Rino, Gan and Tsuzuki, J. Phys Condensed Matter 21 (2009) 095002
</P>
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