Natural states of interacting systems and their use for the calculation of intermolecular forces. II. Natural states in the asymptotic 1/R expansion

The asymptotic natural states i.e. the natural states of the subsystems in the various terms of the asymptotic expansion of the primitive wavefunction of the supersystem are defined. Integro-differential equations are derived that allow the direct calculation of these asymptotic natural states and from them, the terms C₆, C₈ etc. in the asymptotic 1/R expansion of the interaction energy. A pictorial interpretation of these states is given and the rapid convergence of the expansion in natural states is explained. The transferability of certain matrix elements is demonstrated and a virtually exact combination rule for the calculation of the C_k coefficients between different atoms from those for like atoms is obtained- The effects of intraatomic electron correlation on the C_k as well as the justification of a core-valence separation are discussed.