  Classical free energies for the cage and prism isomers of water hexamer
computed by the self- consistent phonons (SCP) method and reversible scaling
(RS) method are presented for several flexible water potentials. Both methods
have been augmented with a rotational correction for improved accuracy when
working with clusters. Comparison of the SCP results with the RS results
suggests a fairly broad temperature range over which the SCP approximation can
be expected to give accurate results for systems of water clusters, and
complements a previously reported assessment of SCP. Discrepancies between the
SCP and RS results presented here, and recently published replica exchange
molecular dynamics (REMD) results bring into question the convergence of the
REMD and accompanying replica exchange path integral molecular dynamics
results. In addition to the ever-present specter of unconverged results,
several possible sources for the discrepancy are explored based on inherent
characteristics of the methods used.
