Ambrosio Francesco, Miceli Giacomo, Pasquarello Alfredo
Chaire de Simulation à l'Echelle Atomique (CSEA), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
J Chem Phys. 2015 Dec 28;143(24):244508. doi: 10.1063/1.4938189.
We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H(+)/H2 level defining the standard hydrogen electrode, the OH(-)/OH(∗) level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH(∗) level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall good agreement with experiment.
我们结合从头算分子动力学（MD）模拟和热力学积分方法，研究水溶液中的氧化还原水平。分子动力学模拟采用半局域的Perdew-Burke-Ernzerhof泛函以及考虑范德华（vdW）相互作用的非局域泛函（rVV10）进行。通过三种不同方案确定能带边缘，即从最高占据和最低未占据的Kohn-Sham态的能量、总能差以及态密度的线性外推来确定。结果表明，后一种方法不依赖于系统大小，而前两种方法存在显著的有限尺寸效应。对于氧化还原水平，我们给出了一种类似于晶体材料中缺陷电荷转移水平定义的公式。我们考虑定义标准氢电极的H(+)/H2水平、对应于氢氧根离子氧化的OH(-)/OH(∗)水平以及水脱氢的H2O/OH(∗)水平。尽管液态水会引起较大的结构变化，但范德华相互作用对计算得到的带隙和能带边缘没有产生任何显著的结构影响。对氧化还原水平的影响也很小，因为离子物种的溶剂化性质受范德华相互作用的影响很小。由于电子性质不受潜在结构性质的显著影响，因此对我们分子动力学模拟的构型进行杂化泛函计算是合理的。计算得到的氧化还原水平作为Fock交换分数α的函数保持不变，再现了先前观察到的缺陷电荷转移水平的一般行为。与实验值的比较显示出非常好的一致性。不同的是，能带边缘和带隙随α线性变化。对于α≃0.40，我们得到的带隙、能带边缘位置和氧化还原水平与实验总体上吻合良好。
