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### pair_style reax command

Syntax:

    pair_style reax hbcut precision

- hbcut = hydrogen-bond cutoff (distance units)
- precision = precision for charge equilibration

Examples:

    pair_style reax
    pair_style reax 10.0 1.0e-5
    pair_coeff * * ffield.reax 3 1 2 2

Description:

The pair style computes the ReaxFF potential of van Duin, Goddard and co-workers. ReaxFF uses distance-dependent bond-order functions to represent the contributions of chemical bonding to the potential energy. There is more than one version of ReaxFF. The version implemented in LAMMPS uses the functional forms documented in the supplemental information of the following paper: (Chenoweth et al., 2008).

LAMMPS requires that a file called ffield.reax be provided, containing the ReaxFF parameters for each atom type, bond type, etc. The format is identical to the ffield file used by van Duin and co-workers. The filename is also reuqired as an argument in the pair_coeff command. Any value other than ffield.reax will be rejected (see below).

LAMMPS provides several different versions of ffield.reax in its potentials dir, each called potentials/ffield.reax.label. These are documented in potentials/README.reax. The default ffield.reax contains parameterizations for the following elements: C, H, O, N, S. You can use one of the other files in place of it, or obtain a file from elsewhere.

The hbcut and precision settings are optional arguments. If neither is provided, default settings are used: hbcut = 10 (which is Angstroms in real units) and precision = 1.0e-6 (one part in 10^6). If you wish to override either of these defaults, then both settings must be specified.

Use of this pair style requires that a charge be defined for every atom since the potential performs charge equilibration. See the atom_style and read_data commands for details on how to specify charges.

The thermo variable evdwl stores the sum of all the ReaxFF potential energy contributions, with the exception of the Coulombic and charge equilibration contributions which are stored in the thermo variable ecoul. The output of these quantities is controlled by the thermo command.

Only a single pair_coeff command is used with the reax style which specifies a ReaxFF 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 indices = mapping of ReaxFF elements to atom types

The specification of the filename and the mapping of LAMMPS atom types recognized by the ReaxFF is done differently than for other LAMMPS potentials, due to the non-portable difficulty of passing character strings (e.g. filename, element names) between C++ and Fortran.

The filename has to be "ffield.reax" and it has to exist in the directory you are running LAMMPS in. This means you cannot prepend a path to the file in the potentials dir. Rather, you should copy that file into the directory you are running from. If you wish to use another ReaxFF potential file, then name it "ffield.reax" and put it in the directory you run from.

In the ReaxFF potential file, near the top, is a section that contains element names, each with a couple dozen numeric parameters. The default ffield.reax provided with LAMMPS lists 5 elements: C, H, O, N, S. Think of these as numbered 1 to 6. Each of the N indices you specify for the N atom types of LAMMPS atoms must be an integer from 1 to 6. Atoms with LAMMPS type 1 will be mapped to whatever element you specify as the first index value, etc.

In the pair_coeff example above, the LAMMPS simulation has 4 atoms types and they are set as follows:

    type 1 = O
    type 2 = C
    type 3 = H
    type 4 = H

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

This pair style does not support the pair_modify mix, 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 pair style is part of the "reax" package. It is only enabled if LAMMPS was built with that package, which also requires the REAX library be built and linked with LAMMPS. See the Making LAMMPS section for more info.

The ffield.reax potential file provided with LAMMPS in the potentials directory is parameterized for real units. You can use the ReaxFF 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't use "real" units. This would be somewhat tricky, so contact the LAMMPS authors if you wish to do this.

Related commands:

pair_coeff

Default: none

(Chenoweth) Chenoweth, van Duin and Goddard III, Journal of Physical Chemistry A, 112, 1040-1053 (2008).