# fix acks2/reaxff command

Accelerator Variants: *acks2/reaxff/kk*

## Syntax

``` LAMMPS
fix ID group-ID acks2/reaxff Nevery cutlo cuthi tolerance params args
```

-   ID, group-ID are documented in [fix](fix) command

-   acks2/reaxff = style name of this fix command

-   Nevery = perform ACKS2 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

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

        keyword = *maxiter*
          *maxiter* N = limit the number of iterations to *N*

## Examples

``` LAMMPS
fix 1 all acks2/reaxff 1 0.0 10.0 1.0e-6 reaxff
fix 1 all acks2/reaxff 1 0.0 10.0 1.0e-6 param.acks2 maxiter 500
```

## Description

Perform the atom-condensed Kohn\--Sham DFT to second order (ACKS2)
charge equilibration method as described in
[(Verstraelen)](Verstraelen). ACKS2 impedes unphysical long-range charge
transfer sometimes seen with QEq (e.g., for dissociation of molecules),
at increased computational cost. It is typically used in conjunction
with the ReaxFF force field model as implemented in the [pair_style
reaxff](pair_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
acks2/reaxff, see the [(O\'Hearn)](O'Hearn) paper.

The ACKS2 method minimizes the electrostatic energy of the system by
adjusting the partial charge on individual atoms based on interactions
with their neighbors. It requires some parameters for each atom type. If
the *params* setting above is the word \"reaxff\", then these are
extracted from the [pair_style reaxff](pair_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 performing QeQ with a non-ReaxFF potential. The lines should
be formatted as follows:

    bond_softness
    itype chi eta gamma bcut

where the first line is the global parameter *bond_softness*. The
remaining 1 to Ntypes lines include *itype*, the atom type from 1 to
Ntypes, *chi*, the electronegativity in eV, *eta*, the self-Coulomb
potential in eV, *gamma*, the valence orbital exponent, and *bcut*, the
bond cutoff distance. Note that these 4 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 $\AA$, eV, and electronic charge.

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

:::: note
::: title
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](restart). This fix computes a global scalar (the number of
iterations) for access by various [output commands](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command.

This fix is invoked during [energy minimization](minimize).

------------------------------------------------------------------------

Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the [Accelerator packages](Speed_packages)
page. The accelerated styles take the same arguments and should produce
the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and
OPT packages, respectively. They are only enabled if LAMMPS was built
with those packages. See the [Build package](Build_package) page for
more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the [-suffix command-line
switch](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_packages) page for more
instructions on how to use the accelerated styles effectively.

------------------------------------------------------------------------

## Restrictions

This fix is part of the REAXFF package. It is only enabled if LAMMPS was
built with that package. See the [Build package](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 less than $10~\AA$.

This fix may be used in combination with [fix efield](fix_efield) and
will apply the external electric field during charge equilibration, but
there may be only one fix efield instance used, it may only use a
constant electric field, and the electric field vector may only have
components in non-periodic directions.

## Related commands

[pair_style reaxff](pair_reaxff), [fix qeq/reaxff](fix_qeq_reaxff)

## Default

maxiter 200

------------------------------------------------------------------------

::: {#O'Hearn}
**(O\'Hearn)** O\'Hearn, Alperen, Aktulga, SIAM J. Sci. Comput., 42(1),
C1\--C22 (2020).
:::

::: {#Verstraelen}
**(Verstraelen)** Verstraelen, Ayers, Speybroeck, Waroquier, J. Chem.
Phys. 138, 074108 (2013).
:::