# 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• General parameter 27: the currently reserved $$p_{elho}$$ parameter in the unpublished electron-hole interaction equation (which is still highly experimental and is subject to change),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] (1, 2) 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