SPECTRAL THEORY OF PHYSICAL AND CHEMICAL-BINDING

A spectral method which provides unified quantum mechanical descriptio ns of both physical and chemical binding phenomena is reported for con structing the adiabatic electronic potential energy surfaces of aggreg ates of atoms or other interacting fragments. The formal development, based on use of a direct product of complete sets of atomic spectral e igenstates and the pairwise-additive nature of the total Hamiltonian m atrix in this basis, is seen to be exact when properly implemented and to provide a separation theorem for N-body interaction energies in te rms of response matrices which can be calculated once and for all for atoms and other fragments of interest. Its perturbation theory expansi on provides a generalization of familiar (Casimir-Polder) second-order pairwise-additive and (Axilrod-Teller) third-order nonadditive intera ction energies, expressions which are recovered explicitly in the long -range-dipole expansion limit. A program of ab initio computational im plementation of the formal. development is described on the basis of u se of optimal (Stieltjes) representations of complete sets of discrete and continuum atomic spectral states, which provide corresponding fin ite-matrix representations of the Hamiltonian. The widely employed pai rwise-additive approximation to nonbonded N-body interaction energies is obtained from these implementations in appropriate limits. Addition ally, the development clarifies and extends rigorously diatomics-in-mo lecules approaches to potential-surface construction for bonding situa tions, includes the effects of state mixing and charge distortion miss ing from semiempirical and perturbation approximations commonly employ ed in theoretical studies of collision broadening and trapped-radical spectroscopy, and encompasses and demonstrates equivalences among thes e apparently dissimilar approaches in appropriate limits. Large non-pa irwise-additive contributions to the lowest-lying potential energy sur faces are found in illustrative studies of the structure and spectra o f physically bound Na-Ar-N cryogenic clusters.