Potential, intermolecular repulsive branch

Fig. 12.1. Schematic illustration of a van der Waals potential as a function of the intermolecular bond distance R measured from the atom X to the center-of-mass of the diatom, I2 in the present case. The orientation angle 7 and the intramolecular I2 bond distance are fixed. The potential parameters governing the long-range attractive and the short-range repulsive branches, namely A, a, and C, depend, in principle, on r and 7.

The weakest spot of empirical intermolecular potentials is perhaps the repulsive branch, which is always ill-determined since the repulsive regime is seldom accessed in equilibrium crystal stmctures at room pressure. The case is different in liquids, where the high molecular mobility occasionally brings molecules into collision. Atom-atom intermolecular energy curves for liquids are usually calibrated separately from those for crystals. 

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