ReaxFF

To be able to deal with bond breaking and formation whilst having only 1 single atom type for each element, ReaxFF is a fairly complex force field with many parameters. Therefore an extensive training set is necessary covering the relevant chemical phase space, including bond and angle stretches, activation and reaction energies, equation of state, surface energies, and much more. Usually, but not necessarily, the training data is generated with electronic structure methods. In practice, often DFT calculations are used as a pragmatic approach, especially since more accurate functionals are available. For the parameterization of such a complex force field, global optimization techniques offer the best chance to get a parameter set that most closely describes the training data.