HOW DO QUANTUM EFFECTS CHANGE CONCLUSIONS ABOUT HETEROGENEOUS CLUSTERBEHAVIOR-BASED ON CLASSICAL MECHANICS SIMULATIONS

Comparisons of classical and quantum Monte Carlo simulation of SF6-(Ar )(n) and SF6-(Ne)(n) clusters are used to examine whether certain nove l types of behavior seen in classical simulations of SF6-(Ar)(n) and S F6-(Kr)(n) persist when quantum effects are taken into account. For mi xed clusters formed from Ar (and presumably other heavy partners) quan tum effects have little effect on calculated properties, even at very low temperatures, so the cluster-size-dependent preference for solvati on vs phase separation and ''reverse melting'' behavior found in the c lassical simulations may be expected to occur in many heterogeneous sy stems. On the other hand, quantum effects substantially lower the melt ing temperatures of clusters formed with Ne, and (except for a couple of unusually stable stacked isomers) effectively remove the barriers s eparating the maximally-solvated and phase-separated forms, implying t hat the latter will normally not exist. Moreover, for (at least) the S F6-(Ne)(11) species, when quantum effects are taken into account there is little evidence of solidlike behavior down to the lowest temperatu res accessible to our simulation (0.4 K), although classical simulatio ns show a sharp freezing transition at 1.5(+/-0.1) K. Inclusion of thr ee-body triple-dipole Axilrod-Teller-Muto interactions in the overall potential energy has little effect on either quantum or classical Ne c luster simulations. (C) 1998 American Institute of Physics. [S0021-960 6(98)00620-5].