pair_style meam/spline command

pair_style meam/spline/omp command

Syntax

pair_style meam/spline

Examples

pair_style meam/spline
pair_coeff * * Ti.meam.spline Ti
pair_coeff * * Ti.meam.spline Ti Ti Ti

Description

The meam/spline style computes pairwise interactions for metals using a variant of modified embedded-atom method (MEAM) potentials (Lenosky). For a single species (“old-style”) MEAM, the total energy E is given by

\[\begin{split}E & =\sum_{i<j}\phi(r_{ij})+\sum_{i}U(n_{i}) \\ n_{i} & =\sum_{j}\rho(r_{ij})+\sum_{\substack{j<k,\\j,k\neq i}}f(r_{ij})f(r_{ik})g[\cos(\theta_{jik})]\end{split}\]

where \(\rho_i\) is the density at atom I, \(\theta_{jik}\) is the angle between atoms J, I, and K centered on atom I. The five functions \(\phi, U, \rho, f,\) and g are represented by cubic splines.

The meam/spline style also supports a new style multicomponent modified embedded-atom method (MEAM) potential (Zhang), where the total energy E is given by

\[\begin{split}E &= \sum_{i<j}\phi_{ij}(r_{ij})+\sum_{i}U_i(n_{i}) \\ n_{i} & = \sum_{j\ne i}\rho_j(r_{ij})+\sum_{\substack{j<k,\\j,k\neq i}}f_{j}(r_{ij})f_{k}(r_{ik})g_{jk}[\cos(\theta_{jik})]\end{split}\]

where the five functions \(\phi, U, \rho, f,\) and g depend on the chemistry of the atoms in the interaction. In particular, if there are N different chemistries, there are N different U, \(\rho\), and f functions, while there are N(N+1)/2 different \(\phi\) and g functions. The new style multicomponent MEAM potential files are indicated by the second line in the file starts with “meam/spline” followed by the number of elements and the name of each element.

The cutoffs and the coefficients for these spline functions are listed in a parameter file which is specified by the pair_coeff command. Parameter files for different elements are included in the “potentials” directory of the LAMMPS distribution and have a “.meam.spline” file suffix. All of these files are parameterized in terms of LAMMPS metal units.

Note that unlike for other potentials, cutoffs for spline-based MEAM potentials are not set in the pair_style or pair_coeff command; they are specified in the potential files themselves.

Unlike the EAM pair style, which retrieves the atomic mass from the potential file, the spline-based MEAM potentials do not include mass information; thus you need to use the mass command to specify it.

Only a single pair_coeff command is used with the meam/spline style which specifies a 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:

  • filename

  • N element names = mapping of spline-based MEAM elements to atom types

See the pair_coeff doc page for alternate ways to specify the path for the potential file.

As an example, imagine the Ti.meam.spline file has values for Ti (old style). If your LAMMPS simulation has 3 atoms types and they are all to be treated with this potentials, you would use the following pair_coeff command:

pair_coeff * * Ti.meam.spline Ti Ti Ti

The 1st 2 arguments must be * * so as to span all LAMMPS atom types. The three Ti arguments map LAMMPS atom types 1,2,3 to the Ti element in the potential file. If a mapping value is specified as NULL, the mapping is not performed. This can be used when a meam/spline potential is used as part of the hybrid pair style. The NULL values are placeholders for atom types that will be used with other potentials. The old-style potential maps any non-NULL species named on the command line to that single type.

An example with a two component spline (new style) is TiO.meam.spline, where the command

pair_coeff * * TiO.meam.spline Ti O

will map the 1st atom type to Ti and the second atom type to O. Note in this case that the species names need to match exactly with the names of the elements in the TiO.meam.spline file; otherwise an error will be raised. This behavior is different than the old style MEAM files.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Speed packages doc page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package doc page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Speed packages doc page for more instructions on how to use the accelerated styles effectively.

Mixing, shift, table, tail correction, restart, rRESPA info:

This pair style does not support the pair_modify shift, table, and tail options.

The meam/spline pair style does not write its information to binary restart files, since it is stored in an external potential parameter file. Thus, you need to re-specify the pair_style and pair_coeff commands in an input script that reads a restart file.

The meam/spline pair style can only be used via the pair keyword of the run_style respa command. They do not support the inner, middle, outer keywords.

Restrictions

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

This pair style is only enabled if LAMMPS was built with the USER-MISC package. See the Build package doc page for more info.