Lattice dynamics of the ethylene crystal with interaction potentials from a b i n i t i o calculations

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Abstract
The long range (electrostatic, dispersion, induction) and short range (exchange and penetration) interaction energy between ethylene molecules has been calculated by ab initio methods as a function of the molecular orientations and distances. The results, when fitted with an exp‐6‐1 atom–atom potential and used in a harmonic lattice dynamics calculation on the ethylene crystal, yield fair agreement with the experimental structure data, ir and Raman phonon frequencies. Although the fit with the atom–atom potential is reasonably good, some specific deviations from the ab initio results indicate the importance of the effects of chemical bonding on the intermolecular potential (leading to noncentral and nonpairwise additive atom–atom forces). The usual empirical atom–atom potentials are grossly corroborated, their main defect being the neglect or underestimate of electrostatic (quadrupole–quadrupole) interactions.