, where ûi (t ′ ) is the unit vector aligned with axis i at time t ′ . For diffusive motion, the OTCF decays exponentially with time t with a decay constant 2D R,i , where D R,i is the rotational diffusion coefficient of this axis (C i (t) ∼ exp (-2D R,i t)). For benzene, the rotational diffusion coefficients of the out-of-plane axis and an in-plane molecular axis were calculated from an exponential fit to the OTCFs (given in previous experimental and atomistic simulation studies of the dynamics of benzene.