Distinguishing vibrational and structural equilibration contributions to thermal expansion

Geometric attributes of a many-body-system potential energy function sugges t a natural description of that system in terms of inherent structures (loc al potential minima), and intra-basin vibrational displacements away from t hose discrete structures. This description is applied herein to the isobari c thermal expansion property (alpha). Starting with the virial form of the pressure equation of state, two distinct contributions to alpha can be unam biguously identified. The first (alpha(vib)) arises from intra-basin anharm onic thermal vibrations; the second (alpha(str)) seems from thermally induc ed structural shifts in basin occupancy. Only alpha(vib) appears for nondef ective crystals, and for glasses below their T-g's. An approximate analysis for liquid water suggests that alpha(str) alone manifests a Liquid-phase d ensity maximum, thought shifted to slightly higher temperature than the obs erved density maximum at 4 degrees C.