fix qeq/rel/reaxff command — LAMMPS documentation

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# fix qeq/rel/reaxff command

## Syntax

    fix ID group-ID qeq/rel/reaxff Nevery cutlo cuthi tolerance params gfile args

- ID, group-ID are documented in fix command

- qeq/rel/reaxff = style name of this fix command

- Nevery = perform QEqR every this many steps

- cutlo,cuthi = lo and hi cutoff for Taper radius

- tolerance = precision to which charges will be equilibrated

- params = reaxff or a filename

- gfile = the name of a file containing Gaussian orbital exponents

- one or more keywords or keyword/value pairs may be appended

      keyword = scale or maxiter or nowarn
        scale beta = set value of scaling factor beta (determines strength of electric polarization)
        maxiter N = limit the number of iterations to N
        nowarn = do not print a warning message if the maximum number of iterations is reached

## Examples

    fix 1 all qeq/rel/reaxff 1 0.0 10.0 1.0e-6 reaxff exp.qeqr
    fix 1 all qeq/rel/reaxff 1 0.0 10.0 1.0e-6 params.qeqr exp.qeqr scale 1.5 maxiter 500 nowarn

## Description

Added in version 2Apr2025.

This fix implements the QEqR method (Lalli) for charge equilibration, which differs from the QEq charge equilibration method (Rappe and Goddard) only in how external electric fields are accounted for. This fix therefore raises a warning when used without fix efield since fix qeq/reaxff should be used when no external electric field is present. Charges are computed with the QEqR method by minimizing the electrostatic energy of the system in the same way as the QEq method but where the absolute electronegativity, \(\chi_i\), of each atom in the QEq method is replaced with an effective electronegativity given by

\[\chi_{\mathrm{r}i} = \chi_i + \frac{\sum_{j=1}^{N} \beta(\phi_i - \phi_j) S_{ij}} {\sum_{m=1}^{N}S_{im}},\]

where \(N\) is the number of atoms in the system, \(\beta\) is a scaling factor, \(\phi_i\) and \(\phi_j\) are the electric potentials at the positions of atoms \(i\) and \(j\) due to the external electric field and \(S_{ij}\) is the overlap integral between atoms \(i\) and \(j\). This formulation is advantageous over the method used by fix qeq/reaxff to account for an external electric field in that it permits periodic boundaries in the direction of an external electric field and in that it does not worsen long-range charge transfer seen with QEq. See Lalli for further details.

This fix is typically used in conjunction with the ReaxFF force field model as implemented in the pair_style reaxff command, but it can be used with any potential in LAMMPS, so long as it defines and uses charges on each atom. For more technical details about the charge equilibration performed by fix qeq/rel/reaxff, which is the same as in fix qeq/reaxff except for the use of \(\chi_{\mathrm{r}i}\), please refer to (Aktulga). To be explicit, fix qeq/rel/reaxff replaces \(\chi_k\) of eq. 3 in (Aktulga) with \(\chi_{\mathrm{r}k}\) when an external electric field is applied.

This fix requires the absolute electronegativity, \(\chi\), in eV, the self-Coulomb potential, \(\eta\), in eV, and the shielded Coulomb constant, \(\gamma\), in \(\AA^{-1}\). If the params setting above is the word “reaxff”, then these are extracted from the pair_style reaxff 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 using this fix with a non-ReaxFF potential. Each line should be formatted as follows, ensuring that the parameters are given in units of eV, eV, and \(\AA^{-1}\), respectively:

    itype chi eta gamma

where itype is the atom type from 1 to Ntypes. Note that eta is defined here as twice the eta value in the ReaxFF file.

The overlap integrals \(S_{ij}\) are computed by using normalized 1s Gaussian type orbitals. The Gaussian orbital exponents, \(\alpha\), that are needed to compute the overlap integrals are taken from the file given by gfile. This file must contain one line for each atom type and provide the Gaussian orbital exponent for each atom type in units of inverse square Bohr radius. Each line should be formatted as follows:

    itype alpha

Empty lines or any text following the pound sign (#) are ignored. An example gfile for a system with two atom types is

    # An example gfile. Exponents are taken from Table 2.2 of Chen, J. (2009).
    # Theory and applications of fluctuating-charge models.
    # The units of the exponents are 1 / (Bohr radius)^2 .
    1  0.2240  # O
    2  0.5434  # H

The optional scale keyword sets the value of \(\beta\) in the equation for \(\chi_{\mathrm{r}i}\). The default value is 1.0.

The optional maxiter keyword allows changing the max number of iterations in the linear solver. The default value is 200.

The optional nowarn keyword silences the warning message printed when the maximum number of iterations is reached. This can be useful for comparing serial and parallel results where having the same fixed number of iterations is desired, which can be achieved by using a very small tolerance and setting maxiter to the desired number of iterations.

Note

In order to solve the self-consistent equations for electronegativity equalization, LAMMPS imposes the additional constraint that all the charges in the fix group must add up to zero. The initial charge assignments should also satisfy this constraint. LAMMPS will print a warning if that is not the case.

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

No information about this fix is written to binary restart files. This fix computes a global scalar (the number of iterations) and a per-atom vector (the effective electronegativity), which can be accessed 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 REAXFF package. It is only enabled if LAMMPS was built with that package. See the Build package page 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 smaller than the non-bonded cutoff radius, which is typically \(10~\AA\) for ReaxFF simulations.

This fix may be used in combination with fix efield and will apply the external electric field during charge equilibration, but there may be only one fix efield instance used and the electric field must be applied to all atoms in the system. Consequently, fix efield must be used with group-ID all and must not be used with the keyword region. Equal-style variables can be used for electric field vector components without any further settings. Atom-style variables can be used for spatially-varying electric field vector components, but the resulting electric potential must be specified as an atom-style variable using the potential keyword for fix efield.

## Related commands

pair_style reaxff, fix qeq/reaxff, fix acks2/reaxff, fix qtpie/reaxff

## Default

scale = 1.0 and maxiter = 200

(Lalli) Lalli and Giusti, Journal of Chemical Physics, 162, 174311 (2025).

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

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