Theory of Long-Range Interatomic Forces. III. First-Order Interaction Energies in the Coupled Representation

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Abstract
The effect of the spin-orbit coupling on the long-range interatomic forces is computed. The representation is diagonal in the spin-orbit Hamiltonian of the two interacting atoms, and the electrostatic perturbation quantizes the total angular momentum J along the internuclear axis. The theory is applied to the interaction of two atoms in the one-electron approximation. In particular, resonance interactions of normal and excited atoms, and the interaction energies between atoms in excited states are calculated. The use of a coupled representation is valid only as long as the electrostatic interaction is a true perturbation on the system. To investigate the transition from a representation in which spin effects are neglected to the one which includes the spin-orbit coupling correctly, a more general perturbation method is formulated.