c7170a4296
git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14117 f3b2605a-c512-4ea7-a41b-209d697bcdaa
212 lines
8.5 KiB
HTML
212 lines
8.5 KiB
HTML
<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 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>
|
|
</HTML>
|