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.

An Efficient Approach for Studies of Water Clusters Using Diffusion Monte Carlo.

An efficient and accurate approach for using diffusion Monte Carlo (DMC) to study molecular clusters is described and applied to an investigation of (H2O)n with n = 1, 2, 3. In this approach, importance sampled DMC is used to describe the intramolecular degrees of freedom of the water monomers. The intermolecular degrees of freedom are treated using standard unguided DMC approaches. This hybrid approach allows us to use significantly smaller simulations to obtain converged zero-point energies and ground state wave functions than would be required for a fully unguided simulation of the same system. We demonstrate the efficiency, accuracy, and utility of this approach through a study of nine isotopologues of water trimer, focusing on how the ground state probability amplitude for these clusters samples the various low-energy minima on the potential surface.

Evaluation of Matrix Elements Using Diffusion Monte Carlo Wave Functions.

Approaches for using diffusion Monte Carlo (DMC) to evaluate matrix elements involving two vibrational wave functions are explored. In the first part of this study, overlaps between a wave function obtained using DMC and one that can be calculated analytically are evaluated. In this case, the analytical wave function is used as the guiding function for an importance sampled DMC simulation. The accuracy of the calculated overlaps is found to depend strongly on the accuracy of the calculated descendant weights, which are obtained in the DMC simulation. While a single evaluation of the descendant weights is sufficient for obtaining projected probability amplitudes or expectation values of multiplicative operators, averages of multiple independent evaluations of the descendant weights are required to obtain accurate matrix elements. This approach is investigated for one-dimensional model systems as well as H2CO, H2D+, and D2H+. The approach is extended to the evaluation of matrix elements of the dipole moment operator between the ground state and states with one quantum of excitation in one of the OH stretching vibrations in H3O2-. For these calculations, the wave functions for both the ground and excited states are evaluated using DMC. The described methodology opens the possibility of evaluating matrix elements involving two different states, both of which are obtained using DMC.