USE OF MOLECULAR OVERLAP TO PREDICT INTERMOLECULAR REPULSION IN N-CENTER-DOT-CENTER-DOT-CENTER-DOT-H-O HYDROGEN-BONDS

The overlap of the monomer charge distributions is used to derive atom -atom intermolecular repulsion models for N ... H and N ... O to model the hydrogen bonds formed between methylcyanide/methanol and pyridine /methanol. The use of the overlap approximation allows the repulsion c ontribution to be assigned to specific atomic pairs, and the anisotrop ic form to be derived analytically. The resulting models are tested ag ainst intermolecular perturbation theory calculations of the exchange- repulsion surface, in the regions sampled by these N ... H-O hydrogen bonds in molecular crystal structures. Such models readily reproduce t he repulsion surface within a 5% rms error, with the main error being attributable to the overlap approximation. The anisotropy of the atom- atom repulsion in these hydrogen bonds is small and the expansion conv erges slowly. However, the methodology shows that considering the over lap is a useful method of assigning the repulsion between organic mole cules into atomic contributions and estimating the isotropic and aniso tropic parameters, which can be applied to larger systems where accura te ab initio calculation of the potential energy surface is not feasib le.