A spectral method which provides unified quantum
mechanical descriptions of both physical and chemical
binding phenomena is reported for constructing the adiabatic electronic
potential energy surfaces of aggregates
of atoms or other interacting fragments. The formal development,
based on use of a direct product of complete
sets of atomic spectral eigenstates and the pairwise-additive nature of
the total Hamiltonian matrix in this
basis, is seen to be exact when properly implemented and to provide a
separation theorem for N-body interaction
energies in terms of response matrices which can be calculated once and
for all for atoms and other fragments
of interest. Its perturbation theory expansion provides a
generalization of familiar (Casimir−Polder) second-order pairwise-additive and (Axilrod−Teller) third-order nonadditive
interaction energies, expressions which
are recovered explicitly in the long-range-dipole expansion limit.
A program of ab initio computational
implementation of the formal development is described on the basis of
use of optimal (Stieltjes) representations
of complete sets of discrete and continuum atomic spectral states,
which provide corresponding finite-matrix
representations of the Hamiltonian. The widely employed
pairwise-additive approximation to nonbonded
N-body interaction energies is obtained from these
implementations in appropriate limits. Additionally,
the
development clarifies and extends rigorously diatomics-in-molecules
approaches to potential-surface construction for bonding situations, includes the effects of state mixing and
charge distortion missing from semiempirical
and perturbation approximations commonly employed in theoretical
studies of collision broadening and trapped-radical spectroscopy, and encompasses and demonstrates equivalences
among these apparently dissimilar
approaches in appropriate limits. Large non-pairwise-additive
contributions to the lowest-lying potential energy
surfaces are found in illustrative studies of the structure and spectra
of physically bound Na−Ar
N
cryogenic
clusters.