BEHAVIOR OF ELECTRON-DENSITY FUNCTIONS IN MOLECULAR-INTERACTIONS

When molecules approach each other at distances typical of gas-phase c omplexes or condensed-phase media, it is known that the intermolecular electron density function in the region between the molecules is rela ted to the strength of the intermolecular interaction. We explore this behavior for 50 interaction pairs, and find that, in the interaction region, the total electron density is well represented by the sum of t he density functions of the isolated molecules. The minimum in the ele ctron density function between pairs of interacting molecules is used to estimate the sizes of the molecules. Taken in conjunction with the density additivity in this region, this procedure provides a means of estimating molecular sizes without performing supermolecule calculatio ns. For weakly interacting systems, the distances and density minima i dentified by this procedure are consistent with use of the 0.002 an is odensity surface to define the size and shape of a molecule in condens ed media.