eReaxFF, ACKS2, LG dispersion
*****************************

Note:

  Only the ReaxFF force fields that mention eReaxFF and ACKS2 support
  those features. The LG dispersion correction is only implemented by
  force fields in the "dispersion" subdirectory.


eReaxFF: classical treatment of the explicit electron
=====================================================

eReaxFF calculations treat one or more electrons or holes explicitly
[1].

An eReaxFF calculation **requires the following additional force-field
parameters to be defined** (equation numbers are referring to the
eReaxFF paper [1]):

* The header of the force-field file must begin with the "[ ereaxff
  acks2 ]" string,

* General parameter 27: the currently reserved p_{elho} parameter in
  the unpublished electron-hole interaction equation (which is still
  highly experimental and is subject to change),

* General parameter 37: the Gauss exponent parameter p_{val}
  describing the free electron, Eq(3),

* General parameter 41: a different taper radius for electron and hole
  interactions can be specified here,

* Atomic parameter 24: the \alpha parameter in Eq(2),

* Atomic parameter 27: the \beta parameter in Eq(2),

* Atomic parameter 13: the p_{ij}^{xel2} parameter in Eq(4d),

* Bond parameter 16: the p_{ij}^{xel1} parameter in Eq(4d).

Additionally, since the eReaxFF method is usually coupled to the ACKS2
charge equilibration method, the ACKS2-related parameters should also
be defined.

In the input geometry file, the explicit electrons are called "El" and
the holes are called "Eh".

**The eReaxFF implementation is considered experimental**, so users
are advised to contact Adri van Duin regarding its use.

[1] M. M. Islam et al., *eReaxFF: A Pseudoclassical Treatment of
    Explicit Electrons within Reactive Force Field Simulations*. J.
    Chem. Theory Comput. 12, 3463 (2016)
    https://doi.org/10.1021/acs.jctc.6b00432


ACKS2 charge equilibration
==========================

The ACKS2 charge equilibration scheme has been implemented following
the original paper [2].

**Using the ACKS2 scheme requires a suitable force-field**, which is
recognized by "[ acks2 ]" at the start of the first line of force
field file (note: the spaces around "acks2" are important!).

In addition to the general EEM parameters the ACKS2 scheme needs the
general force-field parameter #35 ("Xamp") and the atomic cut-off
parameter #23 ("softcut").

[2] T. Verstraelen et al. *ACKS2: Atom-condensed Kohn-Sham DFT
    approximated to second order*. J. Chem. Phys. 138 (2013) 074108.
    https://doi.org/10.1063/1.4791569


LG dispersion correction
========================

The LG dispersion correction was implemented following the paper Liu
et al. [3]

The LG dispersion correction is turned on when using a suitable
forcefield, which is recognized by the “[ lgDispersion=1 ]” key in the
file header.

[3] L. Liu at al. *ReaxFF-lg: Correction of the ReaxFF Reactive Force
    Field for London Dispersion, with Applications to the Equations of
    State for Energetic Materials*. J. Phys. Chem. A, 2011, 115 (40),
    pp 11016–11022. https://doi.org/10.1021/jp201599t
