Dopant location in SF6He39,40

Recent quantum Monte Carlo studies of doped helium clusters have yielded different results for the location of the SF6 impurity, despite good agreement on helium density profiles, thus raising the question of wave function bias on structural properties. We present here a systematic analysis of the effect of the trial function on variational and diffusion Monte Carlo (VMC and DMC) results for the ground state of SF6HeN (N=39 and 40). Four different sets of wave functions are used, together with isotropic pairwise potentials. Use of a two-peak term in the He-SF6 wave function to describe the extensive helium structuring induced by the impurity greatly improves the VMC energies and helium densities. For all of the wave functions, the impurity SF6 distribution has its maximum at the cluster center in both VMC and DMC. This result agrees with the conclusion previously presented by Barnett and Whaley, but it contradicts the recent DMC result of Chin and Krotscheck. To explain this discrepancy, we analyze the amount of sampling necessary to ensure a reliable description of the SF6 density. We find that considerably more sampling is required for the impurity than for the helium density or the energy. Similar considerations are shown to affect the extent of structure seen in pure He-N density profiles. (C) 1996 American Institute of Physics.