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.

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引导扩散蒙特卡洛：研究显示大振幅振动运动的分子和离子的方法

蒙特卡罗扩散法提供了一种有效和高效的方法，用于基于势能面计算分子系统的基态性质。已经表明，当分子内和分子间的振动弱耦合时，该方法需要越来越大的集合。我们最近提出了一种扩散蒙特卡洛法制导方法，以解决水簇的这些挑战[Lee，VGM; McCoy，AB J.Phys。化学 甲 2019，123，8063-8070]。在本研究中，我们扩展了这种方法，并将其应用于结合更强的分子离子，特别是CH ₅ ⁺和H ⁺（H ₂ O）_{n = 1–4}。对于质子化水系统，我们证明了为研究（H ₂ O）_n而开发的指导DMC方法可用于描述溶剂化水分子中的OH拉伸和HOH弯曲以及游离OH拉伸。水合氢核。对于氢键的OH，H延伸至H ⁺（H ₂ O）_n的H ₃ O ⁺核，CH延伸至CH ₅ ⁺，我们根据目标离子的瞬时结构开发自适应指导功能。使用这些指导函数，我们证明了我们能够使用合奏大小来获得收敛的零点能量和基态波函数，该合奏大小应为从无导计算中获得类似精度所需大小的10％。