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Molecular packing and chemical association in liquid water simulated using ab initio hybrid Monte Carlo and different exchange-correlation functionals

机译:液态水中分子堆积和化学缔合的从头开始混合蒙特卡罗和不同的交换相关泛函模拟

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摘要

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.
机译:在溶质水合的自由能中,化学贡献由将水分子从配位球中排出所需的自由能给出,堆积贡献由在散装水中产生无溶质配位球(观察体积)所需的自由能给出。以简单的点电荷/扩展(SPC/E)水模型为参考,利用不同的电子密度泛函模拟水自由能的化学和堆积贡献。密度固定在与压力为 1 bar 的 SPC/E 水相对应的值上。化学贡献表明,在300 K下用BLYP模拟的水比在300 K下用修正的PBE(revPBE)功能模拟的水或在350 K下用BLYP和BLYP-D官能一起模拟的水结合得更紧密。研究了观测体积不同半径的堆积贡献。在观测体积中水分子分布预计为高斯的尺寸范围内,堆积贡献预计随观测球的体积成比例。在 300 K 下使用 revPBE 模拟的水和 在 350 K 下使用 BLYP-D 或 BLYP 模拟的水符合这一预期,但结果表明,BLYP 350 K 和 revPBE 300 K 系统中系统尺寸效应的出现时间比 BLYP-D 350 K 或 SPC/E 观察到的更早。指出了这一观察结果对恒压模拟的意义。对于用BLYP模拟的300 K水,在预期的高斯分布的尺寸范围内,我们反而发现了非高斯行为,并且堆积贡献与观测体积的表面积成比例,表明系统中存在异质性。

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