The Classical Separable Potential (CSP) method, which is a mean-field 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.