Collective dynamics and molecular interactions in liquid CO2 by inelastic neutron scatteringand computer simulations

We report an inelastic neutron scattering determination of the dynamic structure factor of liquid carbon dioxide in the wave-vector range 3<Q<17 nm−1 and molecular dynamics simulations performed using several site-site intermolecular interaction models. The comparison of neutron and simulation dynamical spectra allows effective model-potential selection. The good performance of some of the CO2 anisotropic interaction models enables a thorough investigation of the collective properties, taking full advantage of the direct access that simulations provide to the purely translational modes of a molecular liquid. Center-of-mass collective excitations of liquid CO2 can be fully described through a viscoelastic modeling of the second-order memory function if a free Q dependence of the parameters is allowed. The system behaves in a hydrodynamiclike way up to about Q~5 nm−1, where the thermal relaxation has a dominant role. A rather different situation is found at higher Q, ruled by both structural and dynamical effects, namely, the rapid growth of the static structure peak, the increased damping of the Brillouin lines, and the shortening of the relaxation time. Acoustic excitations propagate up to Q~14 nm−1, while beyond this value a transition to overdamped modes takes place.