The orientational-correlation time intercept in liquids

We propose a theory for the zero viscosity (η) or infinite temperature (T) intercept (τ0) in the much used quasihydrodynamic expression τ2=τ0+(const)(η/T) for the second rank orientational correlation time (τ2). Whereas we associate the (η/T) term with the integral over the torque–torque correlation function, we attribute the origin of τ0 to the integral over the cross-correlation function between torques and kinetic or convective terms. We are unable to evaluate the time dependence of this cross-correlation function exactly, but if a Gaussian form is assumed, a negative value is obtained for τ0. Experimentally, τ0 is sometimes positive and sometimes negative, which suggests that a monotonic form for the time dependence of the cross-correlation function might be inadequate. Because we have not been able to evaluate this cross-correlation function our explanation must still be considered a conjecture.