ReaxFF

ReaxFF ( for "reactive force field") is a force field for the simulation of chemical reactions, which is based on the bond order. It was developed by Adri van Duin, William A. Goddard, III et al. was developed at the California Institute of Technology. Traditional Force Fields ( for example AMBER CHARMM ) are not able to model chemical reactions because the harmonic potential or fixed lists bond used can not describe the breakage and the formation of chemical bonds. ReaxFF has no fixed binding lists. Potentials are used instead, which are dependent on the binding order. This ensures that the potential contributions for vanishing bond order (large interatomic distances ) converge to zero.

In the original version of ReaxFF a parameterization for hydrocarbons has been developed. Meanwhile, there is a number of variants that differ by additional or modified potentials, as well as parameter sets.

2001 version

The 2001 published version has been specially developed for the simulation of hydrocarbons. The energy system is composed of the following terms:

.

Except for the terms and all the terms depend on one or more bond orders. The uncorrected bond order between atoms i and j is defined by:

,

To avoid over- coordination atoms in a molecule, the binding systems are subjected to a correction. This correction causes long-range bonds with lower bond order, already have total bond orders close to the valence their participating atoms, a bond order of zero get.

The term corresponds to the binding energies and by

,

With the parameters and where. Despite bond order correction may occur in molecules for over-or under coordination. In the case of coordination over the term provides

A positive energy contribution. And therein is a parameter. About The coordination of atom i is made up by

.

I is the valence of the atom ( for example, 4 for carbon). The term is an additional energy contribution by the resonance of electrons between adjacent atoms unterkoordinierten. In a molecule or solid Atomtripel ijk have an optimum bond angles (equilibrium angle), which depends on the elements, and the chemical environment (for example, CH4 is HCH angle 109.7 °). A deviation from this equilibrium angle leads to a deterioration in the total energy. In ReaxFF this is realized by the term. In contrast to classical force fields that itself depends on the chemical environment ( sum of the bond orders ).