Pair and singlet translational and rotational diffusion in liquid model N₂

Abstract

Molecular dynamics (MD) calculations were performed with 108 2-centre Lennard-Jones (2 LJC) molecules to study pair and singlet diffusion in a molecular liquid. Our recently proposed 2-centre LJ potential was used to model liquid nitrogen up to 2000 bar. For 12 thermodynamic states, we calculated the pair and singlet diffusion coefficients of the rotational and translational motion. The results for the translation resemble those obtained for spherically symmetric particles indicating that the ‘pair diffusion coefficient’ is a little smaller than the doubled self diffusion coefficient. As opposed to this the ‘pair rotational diffusion’ coefficient lies slightly above the doubled rotational self diffusion coefficient. Apparently, for the rotation, the cross terms governing the pair diffusion become negative and thus add to the diffusion process. A comparison with recent experimental estimates of the ratio of translational pair and singlet diffusion coefficients shows disagreement for most of the states of N₂ considered. However, for two states of high pressure, sufficient agreement exists. One possibility is that the theoretical expression exploited by the experimentalists holds only for these high pressure states.