NORMAL-MODE ANALYSIS OF LIQUID CS2 - VELOCITY CORRELATION-FUNCTIONS AND SELF-DIFFUSION CONSTANTS

Normal mode analysis (NMA) is applied in a molecular-dynamics simulati on of liquid CS2, modeled with a potential including internal degrees of freedom. The entire supercooled liquid range, from the glass transi tion at 100 K to melting at 165 K, and the normal liquid from 165 to 2 93 K, are studied at P = 1 atm. The normal modes of the liquid are cla ssified as translation parallel (trans-parallel to) and perpendicular (trans-perpendicular to) to the molecular axis, rotation, symmetric st retch, antisymmetric stretch, and bend. The configuration-averaged den sity of states, (p(omega)), with both stable and unstable modes, is co rrespondingly decomposed into separate contributions (p(gamma)(omega)) , with gamma = trans-parallel to, etc. The trans-parallel to, trans-pe rpendicular to, and rotational velocity correlation functions, and dif fusion constants D-gamma, are shown to be calculable from the same NMA techniques previously developed for atoms, so long as the appropriate (p(gamma)(omega)) is used. Agreement between NMA theory and simulatio n is extremely good for the trans-perpendicular to velocity correlatio n function and for the diffusion constants in the lower temperature ra nge, is good for the trans-parallel to velocity correlation, and is fa ir for the rotational velocity correlation. Anharmonicities within wel ls of the many-body potential are seen to be more important in CS2 tha n in atomic liquids. At higher temperatures the rotational unstable mo des, (p(mu)(rot)(omega)), show a double-peak structure. It is proposed that the separate contributions of anharmonicity and barrier crossing are causing the two peaks, and a possible connection, respectively, w ith the separate beta and alpha relaxation processes, observed in supe rcooled liquids, is suggested. Several other aspects of liquid-state N MA, including connections with spectroscopic measurements, are briefly considered.