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### fix qeq/reax command

Syntax:

    fix ID group-ID qeq/reax Nevery cutlo cuthi tolerance params

- ID, group-ID are documented in fix command
- qeq/reax = style name of this fix command
- Nevery = perform QEq every this many steps
- cutlo,cuthi = lo and hi cutoff for Taper radius
- tolerance = precision to which charges will be equilibrated
- params = reax/c or a filename

Examples:

    fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c
    fix 1 all qeq/reax 1 0.0 10.0 1.0e-6 param.qeq

Description:

Perform the charge equilibration (QEq) method as described in (Rappe and Goddard) and formulated in (Nakano). It is typically used in conjunction with the ReaxFF force field model as implemented in the pair_style reax/c command, but it can be used with any potential in LAMMPS, so long as it defines and uses charges on each atom. The fix qeq/comb command should be used to perform charge equliibration with the COMB potential. For more technical details about the charge equilibration performed by fix qeq/reax, see the (Aktulga) paper.

The QEq method minimizes the electrostatic energy of the system by adjusting the partial charge on individual atoms based on interactions with their neighbors. It reqires some parameters for each atom type. If the params setting above is the word "reax/c", then these are extracted from the pair_style reax/c command and the ReaxFF force field file it reads in. If a file name is specified for params, then the parameters are taken from the specified file and the file must contain one line for each atom type. The latter form must be used when performing QeQ with a non-ReaxFF potential. Each line should be formatted as follows:

    itype chi eta gamma

where itype is the atom type from 1 to Ntypes, chi denotes the electronegativity in eV, eta denotes the self-Coulomb potential in eV, and gamma denotes the valence orbital exponent. Note that these 3 quantities are also in the ReaxFF potential file, except that eta is defined here as twice the eta value in the ReaxFF file. Note that unlike the rest of LAMMPS, the units of this fix are hard-coded to be A, eV, and electronic charge.

Restart, fix_modify, output, run start/stop, minimize info:

No information about this fix is written to binary restart files. No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command.

This fix is invoked during energy minimization.

Restrictions:

This fix is part of the USER-REAXC package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

This fix does not correctly handle interactions involving multiple periodic images of the same atom. Hence, it should not be used for periodic cell dimensions less than 10 angstroms.

Related commands:

pair_style reax/c

Default: none

(Rappe) Rappe and Goddard III, Journal of Physical Chemistry, 95, 3358-3363 (1991).

(Nakano) Nakano, Computer Physics Communications, 104, 59-69 (1997).

(Aktulga) Aktulga, Fogarty, Pandit, Grama, Parallel Computing, 38, 245-259 (2012).