LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

### fix qeq/comb command

Syntax:

    fix ID group-ID qeq/comb Nevery precision keyword value ...

- ID, group-ID are documented in fix command
- qeq/comb = style name of this fix command
- Nevery = perform charge equilibration every this many steps
- precision = convergence criterion for charge equilibration
- zero or more keyword/value pairs may be appended
- keyword = file

        file value = filename
          filename = name of file to write QEQ equilibration info to

Examples:

    fix 1 surface qeq/comb 10 0.0001

Description:

Perform charge equilibration (QeQ) in conjunction with the COMB (Charge-Optimized Many-Body) potential as described in (COMB_1) and (COMB_2). It performs the charge equilibration portion of the calculation using the so-called QEq method, whereby the charge on each atom is adjusted to minimize the energy of the system. This fix can only be used with the COMB potential; see the fix qeq/reax command for a QeQ calculation that can be used with any potential.

Only charges on the atoms in the specified group are equilibrated. The fix relies on the pair style (COMB in this case) to calculate the per-atom electronegativity (effective force on the charges). An electronegativity equalization calculation (or QEq) is performed in an interative fashion, which in parallel requires communication at each iteration for processors to exchange charge information about nearby atoms with each other. See Rappe_and_Goddard and Rick_and_Stuart for details.

During a run, charge equilibration is peformed every Nevery time steps. Charge equilibration is also always enforced on the first step of each run. The precision argument controls the tolerance for the difference in electronegativity for all atoms during charge equilibration. Precision is a trade-off between the cost of performing charge equilibration (more iterations) and accuracy.

If the file keyword is used, then information about each equilibration calculation is written to the specifed file.

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

This fix produces a per-atom vector which can be accessed by various output commands. The vector stores the gradient of the charge on each atom. The per-atom values be accessed on any timestep.

No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions:

This fix command currently only supports pair style comb.

Related commands:

pair_style comb

Default:

No file output is performed.

(COMB_1) J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),

(COMB_2) T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 81, 125328 (2010).

(Rappe_and_Goddard) A. K. Rappe, W. A. Goddard, J Phys Chem 95, 3358 (1991).

(Rick_and_Stuart) S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys 101, 16141 (1994).