How good is the second cumulant approximation to the reorientational correlation functions in liquid?

Gburski Z, Stassen H, Kachel A, Dorfmüller T. How good is the second cumulant approximation to the reorientational correlation functions in liquid? Journal of Molecular Structure. 1997;410-411:503-506.A general relation linking the angular velocity and reorientational correlation functions is known but it is only of formal character and low practical use. The second cumulant approximation for linear or symmetric top molecules simplifies the calculations to an extent which makes the problem accessible to mathematical analysis and physical interpretation. The question of the quality of this approximation becomes crucial in this context. We used the molecular dynamics (MD) simulations method to calculate both the angular velocity and reorientational correlation functions for liquid carbonyl sulphide (OCS). The relation between these correlation functions, predicted by the second cumulant approximation, vias then carefully tested. (C) 1997 Elsevier Science B.V