HOW GOOD IS THE 2ND CUMULANT APPROXIMATION TO THE REORIENTATIONAL CORRELATION-FUNCTIONS IN LIQUID

A general relation linking the angular velocity and reorientational co rrelation functions is known but it is only of formal character and lo w practical use. The second cumulant approximation for linear or symme tric top molecules simplifies the calculations to an extent which make s the problem accessible to mathematical analysis and physical interpr etation. The question of the quality of this approximation becomes cru cial in this context. We used the molecular dynamics (MD) simulations method to calculate both the angular velocity and reorientational corr elation functions for liquid carbonyl sulphide (OCS). The relation bet ween these correlation functions, predicted by the second cumulant app roximation, vias then carefully tested. (C) 1997 Elsevier Science B.V.