MESA+ Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands.
Dipartimento di Chimica e Chimica Industriale, Università di Pisa , Via Giuseppe Moruzzi 13, 56124 Pisa, Italy.
J Chem Theory Comput. 2016 Apr 12;12(4):1674-83. doi: 10.1021/acs.jctc.6b00044. Epub 2016 Mar 22.
We present for the first time a quantum mechanics/molecular mechanics scheme which combines quantum Monte Carlo with the reaction field of classical polarizable dipoles (QMC/MMpol). In our approach, the optimal dipoles are self-consistently generated at the variational Monte Carlo level and then used to include environmental effects in diffusion Monte Carlo. We investigate the performance of this hybrid model in describing the vertical excitation energies of prototypical small molecules solvated in water, namely, methylenecyclopropene and s-trans acrolein. Two polarization regimes are explored where either the dipoles are optimized with respect to the ground-state solute density (polGS) or different sets of dipoles are separately brought to equilibrium with the states involved in the electronic transition (polSS). By comparing with reference supermolecular calculations where both solute and solvent are treated quantum mechanically, we find that the inclusion of the response of the environment to the excitation of the solute leads to superior results than the use of a frozen environment (point charges or polGS), in particular, when the solute-solvent coupling is dominated by electrostatic effects which are well recovered in the polSS condition. QMC/MMpol represents therefore a robust scheme to treat important environmental effects beyond static point charges, combining the accuracy of QMC with the simplicity of a classical approach.
我们首次提出了一种量子力学/分子力学方案，该方案将量子蒙特卡罗与经典极化偶极子的反应场（QMC/MMpol）相结合。在我们的方法中，最优偶极子在变分蒙特卡罗水平上自洽生成，然后用于在扩散蒙特卡罗中包含环境效应。我们研究了该混合模型在描述水溶剂中小分子原型的垂直激发能方面的性能，即亚甲基环丙烷和反式丙烯醛。探索了两种极化状态，一种是根据基态溶质密度（polGS）优化偶极子，另一种是分别用与电子跃迁相关的状态平衡不同的偶极子（polSS）。通过与参考超分子计算进行比较，其中溶质和溶剂都被量子力学处理，我们发现，与使用冻结环境（点电荷或 polGS）相比，包括环境对溶质激发的响应可以获得更好的结果，特别是当溶质-溶剂耦合主要由静电效应主导时，在 polSS 条件下可以很好地恢复这些效应。因此，QMC/MMpol 代表了一种处理重要环境效应的稳健方案，超越了静态点电荷，将 QMC 的准确性与经典方法的简单性相结合。
