Normal mode analysis of liquid CS2: Velocity correlation functions and self-diffusion constants

Normal mode analysis (NMA) is applied in a molecular-dynamics simulation of liquid CS₂, modeled with a potential including internal degrees of freedom. The entire supercooled liquid range, from the glass transition at 100 K to melting at 165 K, and the normal liquid from 165 to 293 K, are studied at P=1 atm. The normal modes of the liquid are classified as translation parallel (trans-∥) and perpendicular (trans-⊥) to the molecular axis, rotation, symmetric stretch, antisymmetric stretch, and bend. The configuration-averaged density of states, <ρ(ω)>, with both stable and unstable modes, is correspondingly decomposed into separate contributions <ρ^γ(ω)>, with γ=trans-∥, etc. The trans-∥, trans-⊥, and rotational velocity correlation functions, and diffusion constants D^γ, are shown to be calculable from the same NMA techniques previously developed for atoms, so long as the appropriate <ρ^γ(ω)> is used. Agreement between NMA theory and simulation is extremely good for the trans-⊥ velocity correlation function and for the diffusion constants in the lower temperature range, is good for the trans-∥ velocity correlation, and is fair for the rotational velocity correlation. Anharmonicities within wells of the many-body potential are seen to be more important in CS₂ than in atomic liquids. At higher temperatures the rotational unstable modes, <ρ_u^rot(ω)>, 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, with the separate β and α relaxation processes, observed in supercooled liquids, is suggested. Several other aspects of liquid-state NMA, including connections with spectroscopic measurements, are briefly considered.