RESOLVING VIBRATIONAL AND STRUCTURAL CONTRIBUTIONS TO ISOTHERMAL COMPRESSIBILITY

The well-known and general ''compressibility theorem'' for pure substa nces relates kappa(T) = -(partial derivative ln V/partial derivative P )(N,T) to a spatial integral involving the pair correlation function g ((2)). The isochoric inherent structure formalism for condensed phases separates g(2) into two fundamentally distinct contributions: a gener ally anharmonic vibrational part, and a structural relaxation part. On ly the former determines kappa(T) for low-temperature crystals, but bo th operate in the liquid phase. As a supercooled liquid passes downwar d in temperature through a glass transition, the structural contributi on to kappa(T) switches off to produce the experimentally familiar dro p in this quantity. The Kirkwood-Buff solution theory forms the starti ng point for extension to mixtures,with electroneutrality conditions c reating simplifications in the case of ionic systems. (C) 1998 America n Institute of Physics. [S0021-9606(98)50634-4].