pair vashishta/table

</div>

<div class="section" id="pair-style-vashishta-omp-command">

<h1>pair_style vashishta/omp command</h1>

</div>

<div class="section" id="pair-style-vashishta-table-command">

<h1>pair_style vashishta/table command</h1>

</div>

<div class="section" id="pair-style-vashishta-table-omp-command">

<h1>pair_style vashishta/table/omp command</h1>

<div class="section" id="syntax">

<h2>Syntax</h2>

<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">vashishta</span>

</pre></div>

</div>

<pre class="literal-block">

pair_style style args

</pre>

<ul class="simple">

<li>style = <em>vashishta</em> or <em>vashishta/table</em> or <em>vashishta/omp</em> or <em>vashishta/table/omp</em></li>

<li>args = list of arguments for a particular style</li>

</ul>

<pre class="literal-block">

<em>vashishta</em> or <em>vashishta/omp</em> args = none

<em>vashishta/table</em> or <em>vashishta/table/omp</em> args = Ntable cutinner

  Ntable = # of tabulation points

  cutinner = tablulate from cutinner to cutoff

</pre>

</div>

<div class="section" id="examples">

<h2>Examples</h2>

<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">vashishta</span>

<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">SiC</span><span class="o">.</span><span class="n">vashishta</span> <span class="n">Si</span> <span class="n">C</span>

</pre></div>

</div>

<pre class="literal-block">

pair_style vashishta

pair_coeff * * SiC.vashishta Si C

</pre>

<pre class="literal-block">

pair_style vashishta/table 100000 0.2

pair_coeff * * SiC.vashishta Si C

</pre>

</div>

<div class="section" id="description">

<h2>Description</h2>

<p>The <em>vashishta</em> 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 class="reference internal" href="#vashishta1990"><span class="std std-ref">Vashishta1990</span></a>,

SiC <a class="reference internal" href="#vashishta2007"><span class="std std-ref">Vashishta2007</span></a>,

and InP <a class="reference internal" href="#branicio2009"><span class="std std-ref">Branicio2009</span></a>.</p>

<p>The <em>vashishta</em> and <em>vashishta/table</em> styles compute the combined

2-body and 3-body family of potentials developed in the group of Priya

Vashishta and collaborators.  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 class="reference internal" href="#vashishta1990"><span class="std std-ref">Vashishta1990</span></a>, SiC

<a class="reference internal" href="#vashishta2007"><span class="std std-ref">Vashishta2007</span></a>, and InP <a class="reference internal" href="#branicio2009"><span class="std std-ref">Branicio2009</span></a>.</p>

<p>The potential for the energy U of a system of atoms is</p>

<img alt="_images/pair_vashishta.jpg" class="align-center" src="_images/pair_vashishta.jpg" />

<p>where we follow the notation used in <a class="reference internal" href="#branicio2009"><span class="std std-ref">Branicio2009</span></a>.

both zero at <em>rc</em>. The summation over three-body terms

is over all neighbors J and K within a cut-off distance = <em>r0</em>,

where the exponential screening function becomes zero.</p>

<p>Only a single pair_coeff command is used with the <em>vashishta</em> 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,

<p>The <em>vashishta</em> style computes these formulas analytically.  The

<em>vashishta/table</em> style tabulates the analytic values for <em>Ntable</em>

points from cutinner to the cutoff of the potential.  The points are

equally spaced in R^2 space from cutinner^2 to cutoff^2.  For the

two-body term in the above equation, a linear interpolation for each

pairwise distance between adjacent points in the table.  In practice

the tabulated version can run 3-5x faster than the analytic version

with with moderate to little loss of accuracy for Ntable values

between 10000 and 1000000. It is not recommended to use less than

5000 tabulation points.</p>

<p>Only a single pair_coeff command is used with either 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 class="simple">

<li>filename</li>

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>

<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="n">SiC</span><span class="o">.</span><span class="n">vashishta</span> <span class="n">Si</span> <span class="n">Si</span> <span class="n">Si</span> <span class="n">C</span>

</pre></div>

</div>

<pre class="literal-block">

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

<li>C</li>

<li>costheta0</li>

</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

<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 class="docutils" />

<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are

<hr class="docutils" />

<div class="section" id="restrictions">

<h2>Restrictions</h2>

<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 class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section for more info.</p>

<p>This pair style requires the <a class="reference internal" href="newton.html"><span class="doc">newton</span></a> setting to be &#8220;on&#8221;

<p>These pair style are part of the MANYBODY package.  They is only

enabled if LAMMPS was built with that package (which it is by

default).  See the <a class="reference internal" href="Section_start.html#start-3"><span class="std std-ref">Making LAMMPS</span></a> section

for more info.</p>

<p>These pair styles requires the <a class="reference internal" href="newton.html"><span class="doc">newton</span></a> setting to be &#8220;on&#8221;

for pair interactions.</p>

<p>The Vashishta potential files provided with LAMMPS (see the

potentials directory) are parameterized for metal <a class="reference internal" href="units.html"><span class="doc">units</span></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

<p>The Vashishta potential files provided with LAMMPS (see the potentials

directory) are parameterized for metal <a class="reference internal" href="units.html"><span class="doc">units</span></a>.  You can

use the Vashishta potential with any LAMMPS units, but you would need

to create your own potential file with coefficients listed in the

appropriate units if your simulation doesn&#8217;t use &#8220;metal&#8221; units.</p>

</div>

<div class="section" id="related-commands">

<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a></p>

<p><strong>Default:</strong> none</p>

<hr class="docutils" />

<p id="vashishta1990"><strong>(Vashishta1990)</strong> P. Vashishta, R. K. Kalia, J. P. Rino, Phys. Rev. B 41, 12197 (1990).</p>

<p id="vashishta2007"><strong>(Vashishta2007)</strong> P. Vashishta, R. K. Kalia, A. Nakano, J. P. Rino. J. Appl. Phys. 101, 103515 (2007).</p>

<p id="branicio2009"><strong>(Branicio2009)</strong> Branicio, Rino, Gan and Tsuzuki, J. Phys Condensed Matter 21 (2009) 095002</p>

<p id="vashishta1990"><strong>(Vashishta1990)</strong> P. Vashishta, R. K. Kalia, J. P. Rino, Phys. Rev. B

41, 12197 (1990).</p>

<p id="vashishta2007"><strong>(Vashishta2007)</strong> P. Vashishta, R. K. Kalia, A. Nakano,

J. P. Rino. J. Appl. Phys. 101, 103515 (2007).</p>

<p id="branicio2009"><strong>(Branicio2009)</strong> Branicio, Rino, Gan and Tsuzuki, J. Phys Condensed

Matter 21 (2009) 095002</p>

</div>

</div>

# calculate the energy volume curve for InP zincblende

# define volume range and filename

variable	ndelta equal 100

variable	volatom_min equal 20.0

variable	volatom_max equal 29.0

variable	evsvolfile string evsvol.dat

# set up cell 

units		metal

boundary	p p p

# setup loop variables for box volume

variable	amin equal ${volatom_min}^(1/3)*2

variable 	delta equal (${volatom_max}-${volatom_min})/${ndelta} 

variable	scale equal (${delta}/v_volatom+1)^(1/3)

# set up 8 atom InP zincblende unit cell

lattice diamond ${amin}

region		box prism &

	 	0 1 &

		0 1 &

		0 1 &

		0 0 0

create_box	2 box

create_atoms	1	box       &

			basis 5 2 &

			basis 6 2 &

			basis 7 2 &

			basis 8 2

mass 		1 114.76

mass 		2 30.98

# choose potential

pair_style 	vashishta/table 100000 0.2

pair_coeff 	* * InP.vashishta In P

# setup neighbor style

neighbor 	1.0 nsq

neigh_modify 	once no every 1 delay 0 check yes

# setup output

thermo_style 	custom step temp pe press vol

thermo_modify 	norm no

variable 	volatom equal vol/atoms

variable 	eatom equal pe/atoms

print 		"# Volume [A^3/atom] Energy [eV/atom]" file ${evsvolfile}

# loop over range of volumes

label 		loop

variable 	i loop ${ndelta}

change_box 	all x scale ${scale} y scale ${scale} z scale ${scale} remap

# calculate energy

# no energy minimization needed for zincblende

run 		0

print 		"${volatom} ${eatom}" append ${evsvolfile}

next 		i

jump 		SELF loop