diff --git a/doc/html/pair_vashishta.html b/doc/html/pair_vashishta.html index 6dd3e02f8f..8e8b48c9c2 100644 --- a/doc/html/pair_vashishta.html +++ b/doc/html/pair_vashishta.html @@ -129,10 +129,30 @@

pair_style vashishta/omp command

+
+
+

pair_style vashishta/table command

+
+
+

pair_style vashishta/table/omp command

Syntax

-pair_style vashishta
+pair_style style args
+
+ +
+vashishta args = none
+vashishta/omp args = none
+vashishta/table args = Ntable cutinner
+  Ntable = # of tabulation points
+  cutinner = tablulate from cutinner to cutoff
+vashishta/table/omp args = Ntable cutinner
+  Ntable = # of tabulation points
+  cutinner = tablulate from cutinner to cutoff
@@ -141,17 +161,20 @@ pair_style vashishta pair_style vashishta pair_coeff * * SiC.vashishta Si C +
+pair_style vashishta/table 100000 0.2
+pair_coeff * * SiC.vashishta Si C
+

Description

-

The vashishta 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 Vashishta1990, -SiC Vashishta2007, +

The vashishta and vashishta/table styles compute 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 +Vashishta1990, SiC Vashishta2007, and InP Branicio2009.

The potential for the energy U of a system of atoms is

_images/pair_vashishta.jpg @@ -163,10 +186,20 @@ tilted by a linear function so that the energy and force are both zero at rc. The summation over three-body terms is over all neighbors J and K within a cut-off distance = r0, where the exponential screening function becomes zero.

-

Only a single pair_coeff command is used with the vashishta 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, +

The vashishta style computes these formulas analytically. The +vashishta/table style tabulates the analytic values for Ntable +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.

+

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:

-

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.

-

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.

-

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 +

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.

+

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.

+

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.

Styles with a gpu, intel, kk, omp, or opt suffix are @@ -302,20 +328,23 @@ if LAMMPS was built with that package (which it is by default). See the Making LAMMPS section for more info.

This pair style requires the newton setting to be “on” for pair interactions.

-

The Vashishta potential files provided with LAMMPS (see the -potentials directory) are parameterized for metal units. -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.

+

The Vashishta potential files provided with LAMMPS (see the potentials +directory) are parameterized for metal units. 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.