The Classical Separable Potential (CSP) method, which is a meanfield approximation to multidimensional quantum dynamics, is applied to the dephasing process of a vibrationally excited HArF molecule in an argon cluster at low temperatures. Dephasing timescales of the order of 1 ps are estimated for dynamics following fundamental excitation of either the H-Ar or the Ar-F stretching mode of HArF. The CSP approach is valid over such timescales, and it is thus a viable approach to quantum simulations of dephasing at low temperatures. Vibrational relaxation is much slower: Quasi-classical molecular dynamics simulations yield a relaxation time around 100 ps for the initial v = 1 Ar-F stretching excitation. Such timescales are beyond the validity range of CSP; therefore, this or similar separable methods are inapplicable for vibrational energy decay.
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