eReaxFF, ACKS2, LG dispersion

Note

Only the ReaxFF force fields that mention eReaxFF and ACKS2 support these 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 ]” string

• General parameter 27: reserved for the \(p_{elho}\) electron-hole interaction parameter

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

• General parameter 41: a separate 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)

If the eReaxFF method is coupled to the ACKS2 charge equilibration method, the ACKS2-related parameters should also be defined. The header of the force field file should begin with “[ ereaxff acks2 ]” in this case.

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

[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 requires the general parameter 35 (“Xamp”) and the atomic parameter 23 (“softcut”) to be set.

[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 of Liu et al. [3]

The LG dispersion correction is enabled when using a suitable force field, 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