A force field is derived from a set of parameters determined from an approximate energy expression and then used to calculate interatomic or intermolecular energies in a classical calculation or simulation of an atomic structure. Most force field methods include pairwise interatomic interactions (e.g., van der Waals, electrostatic), and some include intramolecular interactions (e.g., bond stretch, angle bend) present in polyatomic species. Interaction parameters are adjusted so that results obtained from the force field, such as structural, mechanical, and spectroscopic properties, match those from experiment or quantum mechanical calculations as closely as possible.