Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
J Phys Chem A. 2020 Nov 19;124(46):9567-9577. doi: 10.1021/acs.jpca.0c07181. Epub 2020 Nov 5.
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. 2019, 123, 8063-8070]. In the present study, we extend this approach and apply it to more strongly bound molecular ions, specifically CH and H(HO). For the protonated water systems, we show that the guided DMC approach that was developed for studies of (HO) 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 HO core of H(HO) and the CH stretches in CH, 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.
扩散蒙特卡罗方法为基于势能面计算分子体系基态性质提供了一种有效且高效的方法。当分子内和分子间振动弱耦合时，该方法已被证明需要越来越大的系综。我们最近提出了一种扩散蒙特卡罗的引导变体来解决这些对于水团簇的挑战[Lee，V. G. M.；McCoy，A. B. 2019，123，8063-8070]。在本研究中，我们扩展了这种方法，并将其应用于更强结合的分子离子，特别是 CH 和 H(HO)。对于质子化的水体系，我们表明，为研究（HO）而开发的引导 DMC 方法可用于描述溶剂水分子中的 OH 伸缩和 HOH 弯曲，以及在水合氢离子核中的游离 OH 伸缩。对于 H(HO)中 HO 核中的氢键 OH 伸缩和 CH 伸展，我们基于感兴趣的离子的瞬时结构开发了自适应引导函数。使用这些引导函数，我们证明我们能够使用小至无引导计算所需大小 10%的系综大小获得收敛的零点能和基态波函数。
