Physical Chemistry Institute, University of Zurich, 8057 Zurich, Switzerland.
J Chem Phys. 2010 May 28;132(20):204509. doi: 10.1063/1.3437061.
In the free energy of hydration of a solute, the chemical contribution is given by the free energy required to expel water molecules from the coordination sphere and the packing contribution is given by the free energy required to create the solute-free coordination sphere (the observation volume) in bulk water. With the simple point charge/extended (SPC/E) water model as a reference, we examine the chemical and packing contributions in the free energy of water simulated using different electron density functionals. The density is fixed at a value corresponding to that for SPC/E water at a pressure of 1 bar. The chemical contribution shows that water simulated at 300 K with BLYP is somewhat more tightly bound than water simulated at 300 K with revised PBE (revPBE) functional or at 350 K with the BLYP and BLYP-D functionals. The packing contribution for various radii of the observation volume is studied. In the size range where the distribution of water molecules in the observation volume is expected to be Gaussian, the packing contribution is expected to scale with the volume of the observation sphere. Water simulated at 300 K with the revPBE and at 350 K with BLYP-D or BLYP conforms to this expectation, but the results suggest an earlier onset of system size effects in the BLYP 350 K and revPBE 300 K systems than that observed for either BLYP-D 350 K or SPC/E. The implication of this observation for constant pressure simulations is indicated. For water simulated at 300 K with BLYP, in the size range where Gaussian distribution of occupation is expected, we instead find non-Gaussian behavior, and the packing contribution scales with surface area of the observation volume, suggesting the presence of heterogeneities in the system.
在溶剂的水合自由能中，化学贡献由将水分子从配位球中逐出所需的自由能给出，而堆积贡献由在本体水中创建无溶质的配位球（观测体积）所需的自由能给出。以简单点电荷/扩展（SPC/E）水模型作为参考，我们研究了使用不同电子密度泛函模拟水时的自由能中的化学和堆积贡献。密度固定在对应于 SPC/E 水在 1 巴压力下的值。化学贡献表明，在 300 K 下用 BLYP 模拟的水比在 300 K 下用修订后的 PBE（revPBE）功能或在 350 K 下用 BLYP 和 BLYP-D 功能模拟的水结合得更紧密。研究了不同观测体积半径的堆积贡献。在观测体积中水分子分布预计呈高斯分布的尺寸范围内，堆积贡献预计与观测球体积成比例。用 revPBE 在 300 K 下模拟的水和用 BLYP-D 或 BLYP 在 350 K 下模拟的水符合这一预期，但结果表明 BLYP 在 350 K 和 revPBE 在 300 K 系统中的系统尺寸效应出现得更早，而不是像 BLYP-D 在 350 K 或 SPC/E 那样观察到的那样。这一观察结果对恒压模拟的影响表明。对于在 300 K 下用 BLYP 模拟的水，在预计出现占据的高斯分布的尺寸范围内，我们反而发现了非高斯行为，堆积贡献与观测体积的表面积成比例，这表明系统中存在不均匀性。
