Resolving vibrational and structural contributions to isothermal compressibility

The well-known and general ‘‘compressibility theorem’’ for pure substances relates kT 52(] ln V/]p)N,T to a spatial integral involving the pair correlation function g . The isochoric inherent structure formalism for condensed phases separates g (2) into two fundamentally distinct contributions: a generally anharmonic vibrational part, and a structural relaxation part. Only the former determines kT for low-temperature crystals, but both operate in the liquid phase. As a supercooled liquid passes downward in temperature through a glass transition, the structural contribution to kT switches off to produce the experimentally familiar drop in this quantity. The Kirkwood–Buff solution theory forms the starting point for extension to mixtures, with electroneutrality conditions creating simplifications in the case of ionic systems. © 1998 American Institute of Physics. @S0021-9606~98!50634-4#