Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
J Chem Phys. 2012 Jan 28;136(4):044505. doi: 10.1063/1.3677184.
The relationship is investigated for QM/MM (quantum-mechanical/molecular-mechanical) systems between the fluctuations of the electronic state of the QM subsystem and of the solvation effect due to the QM-MM interaction. The free-energy change due to the electron-density fluctuation around its average is highlighted, and is evaluated through an approximate functional formulated in terms of distribution functions of the many-body coupling (pairwise non-additive) part of the QM-MM interaction energy. A set of QM/MM simulations are conducted in MM water solvent for QM water solute in ambient and supercritical conditions and for QM glycine solute in the neutral and zwitterionic forms. The variation of the electronic distortion energy of the QM solute in the course of QM/MM simulation is then shown to be compensated by the corresponding variation of the free energy of solvation. The solvation free energy conditioned by the electronic distortion energy is further analyzed with its components. It is found that the many-body contribution is essentially equal between the free energy and the average sum of solute-solvent interaction energy.
本文研究了 QM/MM（量子力学/分子力学）体系中 QM 子系统电子态涨落与 QM-MM 相互作用引起的溶剂化效应之间的关系。重点研究了由于电子密度围绕平均值的涨落引起的自由能变化，并通过用 QM-MM 相互作用能的多体耦合（非加和的成对）部分的分布函数来评估该变化。在 MM 水溶剂中，对 QM 水溶质在环境和超临界条件下以及 QM 甘氨酸溶质在中性和两性离子形式下进行了一组 QM/MM 模拟。然后，显示 QM 溶质在 QM/MM 模拟过程中电子变形能的变化被溶剂化自由能的相应变化所补偿。进一步对电子变形能条件下的溶剂化自由能进行了分析，得到了其组成部分。结果发现，多体贡献在自由能和溶质-溶剂相互作用能的平均和之间基本相等。
