Path-integral Monte Carlo simulation of nu3 vibrational shifts for CO2 in (He)n clusters critically tests the He-CO2 potential energy surface.

Path-integral Monte Carlo simulations of the nu(3) vibrational band origin frequency shifts of CO(2) in (He)(n) clusters for n=1-40 show that although only the asymmetric-stretch mode of CO(2) is being excited, the effect of the associated change in the average value of Q(1) cannot be ignored. When this fourth degree of freedom is taken into account, the resulting predicted vibrational frequency shifts are in excellent agreement with experiment across this whole range of cluster size. It is also shown that the quality of predictions obtained from simulations on a given potential energy surface can depend significantly on the choice of the analytic function used to represent it.