Dahlgren Björn, Reif Maria M, Hünenberger Philippe H, Hansen Niels
Laboratory of Physical Chemistry, ETH Zürich, Zürich, Switzerland.
Institute for Molecular Modeling and Simulation, University of Natural Resources and Life Sciences, Vienna, Austria.
J Chem Theory Comput. 2012 Oct 9;8(10):3542-64. doi: 10.1021/ct300260q. Epub 2012 Jul 19.
The raw ionic solvation free energies calculated on the basis of atomistic (explicit-solvent) simulations are extremely sensitive to the boundary conditions and treatment of electrostatic interactions used during these simulations. However, as shown recently [Kastenholz, M. A.; Hünenberger, P. H. J. Chem. Phys.2006, 124, 224501 and Reif, M. M.; Hünenberger, P. H. J. Chem. Phys.2011, 134, 144104], the application of an appropriate correction scheme allows for a conversion of the methodology-dependent raw data into methodology-independent results. In this work, methodology-independent derivative thermodynamic hydration and aqueous partial molar properties are calculated for the Na(+) and Cl(-) ions at P° = 1 bar and T(-) = 298.15 K, based on the SPC water model and on ion-solvent Lennard-Jones interaction coefficients previously reoptimized against experimental hydration free energies. The hydration parameters considered are the hydration free energy and enthalpy. The aqueous partial molar parameters considered are the partial molar entropy, volume, heat capacity, volume-compressibility, and volume-expansivity. Two alternative calculation methods are employed to access these properties. Method I relies on the difference in average volume and energy between two aqueous systems involving the same number of water molecules, either in the absence or in the presence of the ion, along with variations of these differences corresponding to finite pressure or/and temperature changes. Method II relies on the calculation of the hydration free energy of the ion, along with variations of this free energy corresponding to finite pressure or/and temperature changes. Both methods are used considering two distinct variants in the application of the correction scheme. In variant A, the raw values from the simulations are corrected after the application of finite difference in pressure or/and temperature, based on correction terms specifically designed for derivative parameters at P° and T(-). In variant B, these raw values are corrected prior to differentiation, based on corresponding correction terms appropriate for the different simulation pressures P and temperatures T. The results corresponding to the different calculation schemes show that, except for the hydration free energy itself, accurate methodological independence and quantitative agreement with even the most reliable experimental parameters (ion-pair properties) are not yet reached. Nevertheless, approximate internal consistency and qualitative agreement with experimental results can be achieved, but only when an appropriate correction scheme is applied, along with a careful consideration of standard-state issues. In this sense, the main merit of the present study is to set a clear framework for these types of calculations and to point toward directions for future improvements, with the ultimate goal of reaching a consistent and quantitative description of single-ion hydration thermodynamics in molecular dynamics simulations.
基于原子尺度(显式溶剂)模拟计算得到的原始离子溶剂化自由能对这些模拟过程中使用的边界条件和静电相互作用处理极为敏感。然而,正如最近所表明的[卡斯滕霍尔茨,M. A.;许嫩贝格尔,P. H.《化学物理杂志》2006年,124卷,224501期以及赖夫,M. M.;许嫩贝格尔,P. H.《化学物理杂志》2011年,134卷,144104期],应用适当的校正方案可将依赖于方法的原始数据转换为与方法无关的结果。在本工作中,基于SPC水模型以及先前针对实验溶剂化自由能重新优化的离子 - 溶剂 Lennard - Jones 相互作用系数,计算了在P° = 1巴和T(-) = 298.15 K时Na(+)和Cl(-)离子的与方法无关的导数热力学水合及水相偏摩尔性质。所考虑的水合参数为水合自由能和焓。所考虑的水相偏摩尔参数为偏摩尔熵、体积、热容、体积压缩性和体积膨胀性。采用两种替代计算方法来获取这些性质。方法I依赖于两个包含相同数量水分子的水相体系之间平均体积和能量的差异,这两个体系分别是不存在离子和存在离子的情况,以及这些差异随有限压力或/和温度变化的情况。方法II依赖于离子水合自由能的计算,以及该自由能随有限压力或/和温度变化的情况。两种方法在应用校正方案时都考虑了两个不同的变体。在变体A中,基于专门为P°和T(-)处的导数参数设计的校正项,在对压力或/和温度应用有限差分之后对模拟的原始值进行校正。在变体B中,基于适用于不同模拟压力P和温度T的相应校正项,在求导之前对这些原始值进行校正。对应于不同计算方案的结果表明,除了水合自由能本身之外,尚未实现准确的方法独立性以及与即使是最可靠的实验参数(离子对性质)的定量一致性。然而,只有在应用适当的校正方案并仔细考虑标准态问题时,才能实现近似的内部一致性和与实验结果的定性一致性。从这个意义上讲,本研究的主要优点是为这类计算设定了一个清晰的框架,并指出了未来改进的方向,最终目标是在分子动力学模拟中对单离子水合热力学达成一致且定量的描述。