Orientational relaxation of a Fokker–Planck fluid of symmetric top molecules

Orientational correlation functions and correlation times are calculated for a dense fluid of symmetric top molecules using a rotational Fokker–Planck (FP) equation. The method of solution is a frequency domain cumulant expansion. In dense fluids, collisions dominate the dynamics wherein free rotational streaming can be treated perturbatively. Using an approximate resummation of the cumulant series it is shown that orientational correlation times for symmetric tops, rotors, and spherical tops display the structure τl =I⊥/[kBTl (l+i) τJ,⊥] +τJ,⊥(1−hl), where hl is a small correlation which depends on particle shape. The logarithm of the correlation function for long times depends linearly on time with a t=0 intercept which is greater than unity. On the basis of the intercept, one can easily distinguish FP correlation functions from those derived from the Gordon models.