A study of short-range repulsions

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Abstract
A first-order SCF theory is proposed to treat nonbonded interactions of atoms (subsystems) in arbitrary states in order to investigate intermolecular atom–atom repulsions between closed-shell molecules. Applications to 12 homonuclear and 15 heteronuclear pairs of atoms are reported. The repulsions so obtained can always be represented by exponentials V(R) = C exp(−η R). These results are used to establish the systematic dependence of C and η on ε (the highest orbital energy), and to test the validity of combination rules, of which an ’’energy dependent hard core type’’ rule [Eq.(32)] shows the best performance. Ideas emerging from these investigations are then applied to discuss van der Waals radii and to develop a simplified treatment of repulsive parts of intermolecular interactions. A new and generally applicable approach allows for an approximate calculation of repulsive contributions which lead for (N2)2 to errors corresponding to a shift of ≲0.1a0 in the corresponding curves.