# pair_style mliap command

## Syntax

pair_style mliap


## Examples

pair_style mliap model linear InP.mliap.model descriptor sna InP.mliap.descriptor
pair_style mliap model quadratic W.mliap.model descriptor sna W.mliap.descriptor
pair_coeff * * In P


## Description

Pair style mliap provides a general interface to families of machine-learning interatomic potentials. It provides separate definitions of the interatomic potential functional form (model) and the geometric quantities that characterize the atomic positions (descriptor). By defining model and descriptor separately, it is possible to use many different models with a given descriptor, or many different descriptors with a given model. Currently, the pair_style supports just two models, linear and quadratic, and one descriptor, sna, the SNAP descriptor used by pair_style snap, including the linear, quadratic, and chem variants. Work is currently underway to extend the interface to handle neural network energy models, and it is also straightforward to add new descriptor styles.

The pair_style mliap command must be followed by two keywords model and descriptor in either order. A single pair_coeff command is also required. The first 2 arguments must be * * so as to span all LAMMPS atom types. This is followed by a list of N arguments that specify the mapping of MLIAP element names to LAMMPS atom types, where N is the number of LAMMPS atom types.

The model keyword is followed by a model style, currently limited to either linear or quadratic. In both cases, this is followed by a single argument specifying the model filename containing the linear or quadratic coefficients for a set of elements. The model filename usually ends in the .mliap.model extension. It may contain coefficients for many elements. The only requirement is that it contain at least those element names appearing in the pair_coeff command.

The top of the model file can contain any number of blank and comment lines (start with #), but follows a strict format after that. The first non-blank non-comment line must contain two integers:

• nelems = Number of elements

• ncoeff = Number of coefficients

This is followed by one block for each of the nelem elements. Each block consists of ncoeff coefficients, one per line. Note that this format is similar, but not identical to that used for the pair_style snap coefficient file. Specifically, the line containing the element weight and radius is omitted, since these are handled by the descriptor.

The descriptor keyword is followed by a descriptor style, and additional arguments. Currently the only descriptor style is sna, indicating the bispectrum component descriptors used by the Spectral Neighbor Analysis Potential (SNAP) potentials of pair_style snap. The 'p' in SNAP is dropped, because keywords that match pair_styles are silently stripped out by the LAMMPS command parser. A single additional argument specifies the descriptor filename containing the parameters and setting used by the SNAP descriptor. The descriptor filename usually ends in the .mliap.descriptor extension.

The SNAP descriptor file closely follows the format of the pair_style snap parameter file. The file can contain blank and comment lines (start with #) anywhere. Each non-blank non-comment line must contain one keyword/value pair. The required keywords are rcutfac and twojmax. There are many optional keywords that are described on the pair_style snap doc page. In addition, the SNAP descriptor file must contain the nelems, elems, radelems, and welems keywords. The nelems keyword specifies the number of elements provided in the other three keywords. The elems keyword is followed by a list of nelems element names that must include the element names appearing in the pair_coeff command, but can contain other names too. Similarly, the radelems and welems keywords are followed by lists of nelems numbers giving the element radius and element weight of each element. Obviously, the order in which the elements are listed must be consistent for all three keywords.

See the pair_coeff doc page for alternate ways to specify the path for these model and descriptor files.

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

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 with user-specifiable parameters as described above. You never need to specify a pair_coeff command with I != J arguments for this style.

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

This pair style does not write its information to binary restart files, 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.

This pair style can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.

## Restrictions

This style is part of the MLIAP package. It is only enabled if LAMMPS was built with that package. In addition, building LAMMPS with the MLIAP package requires building LAMMPS with the SNAP package. See the Build package doc page for more info.