Guided Diffusion Monte Carlo: A Method for Studying Molecules and Ions That Display Large Amplitude Vibrational Motions

Diffusion Monte Carlo provides an
effective and efficient approach
for calculating ground state properties of molecular systems based
on potential energy surfaces. The approach has been shown to require
increasingly large ensembles when intra- and intermolecular vibrations
are weakly coupled. We recently proposed a guided variant of diffusion
Monte Carlo to address these challenges for water clusters [Lee, V. G. M.; McCoy, A. B. J. Phys.
Chem. A 2019, 123, 8063−8070]. In the present study, we
extend this approach and apply it to more strongly bound molecular
ions, specifically CH5+ and H+(H2O)n=1–4. For the protonated water systems, we
show that the guided DMC approach that was developed for studies of
(H2O)n can be used to describe
the OH stretches and HOH bends in the solvating water molecules, as
well as the free OH stretches in the hydronium core. For the hydrogen
bonded OH stretches in the H3O+ core of H+(H2O)n and the CH stretches
in CH5+,
we develop adaptive guiding functions based on the instantaneous structure
of the ion of interest. Using these guiding functions, we demonstrate
that we are able to obtain converged zero-point energies and ground
state wave functions using ensemble sizes that are as small as 10%
the size that is needed to obtain similar accuracy from unguided calculations.

扩散蒙特卡洛方法提供了一种有效且高效的途径，用以计算基于势能表面的分子系统的基态性质。该方法已证实，在分子内和分子间振动耦合较弱的情况下，需要使用日益增大的集合。我们最近提出了一种引导式的扩散蒙特卡洛方法，用以应对这些挑战，应用于水簇的研究[李，V. G. M.；麦科伊，A. B. J. Phys. Chem. A 2019, 123, 8063−8070]。在本研究中，我们扩展了这一方法，并将其应用于更紧密结合的分子离子，特别是CH5+和H+(H2O)n=1–4。对于质子化水系统，我们表明，为研究(H2O)n而开发的引导DMC方法，可以用来描述溶剂化水分子中的OH伸缩和HOH弯曲，以及水合氢离子核心中的自由OH伸缩。对于H+(H2O)n核心中的氢键OH伸缩和CH5+中的CH伸缩，我们基于目标离子的瞬时结构开发了自适应引导函数。利用这些引导函数，我们证明了我们能够通过比未引导计算获得相似精度所需的集合大小小10%的集合大小，获得收敛的零点能和基态波函数。