Statistical Mechanics of Linear Molecules. I. Potential Energy Functions

Intermolecular potential energy functions for a pair of linear symmetric molecules are considered. The orientation- and distance-dependent molecular properties of pairs of such molecules are expressed in a standard series form. The terms in these series consist of orthonormal polynomials dependent upon the angle multiplied by arbitrary functions of distance. A number of the arbitrary functions are computed numerically for two model interaction energies: the linear Kihara core potential, and a model denoted by the term ``Diatomic'' which was obtained by assuming that each molecule consisted of a pair of atomic interaction centers. Changes in the components of the series for the energy are shown for both models as a function of the length of the molecule. Some conclusions are reached concerning the relative importance of the various angle-dependent terms in the potentials for molecules of a given length. The results obtained for the two models with equivalent values for the parameters are compared, and the adequacy of the spherically symmetric approximation which is often invoked for these functions is discussed.