Some Diabatic (Quasistationary) States of Small Diatomic Systems—Projected Atomic Orbitals

It has been recognized, both from experiment and from theory, that the diatomic potential curves followed by colliding atoms are frequently not those of the conventional stationary adiabatic states. The actual states, which have been named “diabatic,” are most easily understood as adiabatic states which are quasistationary rather than stationary. In the work reported here, several methods (those of Holoeien, Lipsky–Russek, and Feshbach), which were developed for atoms, are applied in an exploratory manner to calculate quasistationary electronic states of some small diatomic systems. The potential‐energy curves calculated are those for ${\mathrm{H}}_2{(}^1{\Sigma }_{\mathrm{g,u}})$ and also HeH⁺(¹Σ), He₂⁺(²Σ_g), and He₂(³Π_u). It was found that the method of Holoeien and of Lipsky and Russek, when applied to molecules, gave results which were satisfactory and physical for all R's, and generally superior to that of Feshbach. A new and simpler method, using projected atomic orbitals in a valence‐bond wavefunction, was also developed. Aside from their direct usefulness for simple processes such as differential elastic scattering and symmetric charge transfer (a numerical application is made to double charge transfer in H^–H⁺ collisions), these diabatic states each form the nucleus of a quasiadiabatic representation of the electronic Hamiltonian, from which inelastic, charge‐changing, and rearrangement collisions involving these systems can be calculated most directly and precisely.