Unfortunately, many experimental NMR results of dihydrobenzene, dihydronapthalene, and dihydroanthracens were interpreted on the preconceived notion that planar forms are unstable as predeicted by the hand-held models and as indicated by solid state structures that were influenced by crystal packing forces. However, no molecule is strain free. Treatment relies heavily on availability of experimental data for parameterization. In the simplest force fields, perhaps a dozen or so parameters are necessary to describe the alkanes. Such a force field gives a reasonable approximation to molecular structures and E differences, but the values obtained from such a force field are clearly inferior to values measured experimentally To better reproduce the available exptal data, further optimization of the parameters or the introduction of more of them by modifying the equations that make up the force field might be considered.
EFF methods are interpolations and sometimes extrapolations of existing data.
Pictorial diagram of various components. Each modification means a reoptimization. Some EFF's are designed to reproduce strictures and E's only. Each of these forces are defined in terms of potential energy functions of the internal coordinated of the molecules that constitute the molecular force field. The function is presumed to have a minimum corresponding to stable equilibrium geometries.