Takahashi Hideaki
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan.
J Chem Phys. 2023 Sep 28;159(12). doi: 10.1063/5.0160465.
In a recent study, we developed a kinetic-energy density functional that can be utilized in orbital-free quantum mechanical/molecular mechanical (OF-QM/MM) simulations. The functional includes the nonlocal term constructed from the response function of the reference system of the QM solute. The present work provides a method to combine the OF-QM/MM with a theory of solutions based on the energy representation to compute the solvation free energy of the QM solute in solution. The method is applied to the calculation of the solvation free energy Δμ of a QM water solute in an MM water solvent. It is demonstrated that Δμ is computed as -7.7 kcal/mol, in good agreement with an experimental value of -6.3 kcal/mol. We also develop a theory to map the free energy δμ due to electron density polarization onto the coordinate space of electrons. The free energy density obtained by the free-energy mapping for the QM water clarifies that each hydrogen atom makes a positive contribution (+34.7 kcal/mol) to δμ, and the oxygen atom gives the negative free energy (-71.7 kcal/mol). It is shown that the small polarization free energy -2.4 kcal/mol is generated as a result of the cancellation of these counteracting energies. These analyses are made possible by the OF-QM/MM approach combined with a statistical theory of solutions.
在最近的一项研究中，我们开发了一种动能密度泛函，可用于无轨道量子力学/分子力学（OF-QM/MM）模拟。该泛函包括由量子力学溶质参考系统的响应函数构建的非局部项。本工作提供了一种将OF-QM/MM与基于能量表示的溶液理论相结合的方法，以计算溶液中量子力学溶质的溶剂化自由能。该方法应用于计算量子力学水溶质在分子力学水溶剂中的溶剂化自由能Δμ。结果表明，计算得到的Δμ为-7.7 kcal/mol，与实验值-6.3 kcal/mol吻合良好。我们还开发了一种理论，将由于电子密度极化引起的自由能δμ映射到电子的坐标空间。通过对量子力学水进行自由能映射得到的自由能密度表明，每个氢原子对δμ做出正贡献（+34.7 kcal/mol），而氧原子给出负自由能（-71.7 kcal/mol）。结果表明，由于这些相互抵消的能量相互抵消，产生了较小的极化自由能-2.4 kcal/mol。这些分析通过将OF-QM/MM方法与溶液统计理论相结合得以实现。
