Amira Sami, Spångberg Daniel, Zelin Viktor, Probst Michael, Hermansson Kersti
Materials Chemistry, The Angström Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Sweden.
J Phys Chem B. 2005 Jul 28;109(29):14235-42. doi: 10.1021/jp050186u.
The optimized geometry and energetic properties of Fe(D2O)n 3+ clusters, with n = 4 and 6, have been studied with density-functional theory calculations and the BLYP functional, and the hydration of a single Fe 3+ ion in a periodic box with 32 water molecules at room temperature has been studied with Car-Parrinello molecular dynamics and the same functional. We have compared the results from the CPMD simulation with classical MD simulations, using a flexible SPC-based water model and the same number of water molecules, to evaluate the relative strengths and weaknesses of the two MD methods. The classical MD simulations and the CPMD simulations both give Fe-water distances in good agreement with experiment, but for the intramolecular vibrations, the classical MD yields considerably better absolute frequencies and ion-induced frequency shifts. On the other hand, the CPMD method performs considerably better than the classical MD in describing the intramolecular geometry of the water molecule in the first hydration shell and the average first shell...second shell hydrogen-bond distance. Differences between the two methods are also found with respect to the second-shell water orientations. The effect of the small box size (32 vs 512 water molecules) was evaluated by comparing results from classical simulations using different box sizes; non-negligible effects are found for the ion-water distance and the tilt angles of the water molecules in the second hydration shell and for the O-D stretching vibrational frequencies of the water molecules in the first hydration shell.
利用密度泛函理论计算和BLYP泛函研究了n = 4和6时Fe(D2O)n 3+团簇的优化几何结构和能量性质，并利用Car-Parrinello分子动力学和相同泛函研究了在室温下单个Fe 3+离子在含有32个水分子的周期性盒子中的水合作用。我们将CPMD模拟的结果与经典MD模拟的结果进行了比较，使用了基于柔性SPC的水模型和相同数量的水分子，以评估这两种MD方法的相对优缺点。经典MD模拟和CPMD模拟给出的Fe-水距离与实验结果吻合良好，但对于分子内振动，经典MD给出的绝对频率和离子诱导频率位移要好得多。另一方面，在描述第一水合层中水分子的分子内几何结构和平均第一壳层……第二壳层氢键距离方面，CPMD方法比经典MD方法表现得要好得多。在第二壳层水的取向方面也发现了两种方法之间的差异。通过比较使用不同盒子大小的经典模拟结果，评估了小盒子大小（32个水分子与512个水分子）的影响；发现对于离子-水距离、第二水合层中水分子的倾斜角以及第一水合层中水分子的O-D伸缩振动频率存在不可忽略的影响。
