Diffusion Monte Carlo Studies on the Detection of Structural Changes in the Water Hexamer upon Isotopic Substitution

The ground-state
structures of water hexamer and several deuterated
variants are studied by using the diffusion Monte Carlo (DMC) method.
We demonstrate that a recently developed guided DMC approach allows
us to study these systems using substantially smaller ensembles than
are required for standard DMC approaches. DMC calculations of the
ground states of (H2O)6 and (D2O)6 using the MB-pol potential with 50 000 walkers and a 1 au time
step show that for (H2O)6 the cage structure
is 51 ± 7 cm–1 lower in energy than the prism
structure. In the case of (D2O)6, the two structures
have nearly equal energies (ΔE = 9 ± 11
cm–1). The structures of the singly substituted,
(H2O)­(D2O)5 and (H2O)5(D2O), variants of water hexamer are also explored
to identify the impact of the location of the unique water molecule
on the relative stability of the possible structure. The effect on
the stability of the hydrogen bond on whether a hydrogen bond has
an OD or OH bond as the donor is also explored.