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

Abstract

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 〈ργ(ω)〉, with γ=trans-∥, etc. The trans-∥, trans-⊥, and rotational velocity correlation functions, and diffusion constants Dγ, are shown to be calculable from the same NMA techniques previously developed for atoms, so long as the appropriate 〈ργ(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, 〈ρurot(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.