Le Breton Guillaume, Bonhomme Oriane, Benichou Emmanuel, Loison Claire
University of Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Villeurbanne, France.
Phys Chem Chem Phys. 2022 Aug 17;24(32):19463-19472. doi: 10.1039/d2cp00803c.
The molecular first hyperpolarizability contributes to second-order optical non-linear signals collected from molecular liquids. For the Second Harmonic Generation (SHG) response, the first hyperpolarizability (2, , ) often depends on the molecular electrostatic environment. This is especially true for water, due to its large second hyperpolarizability (2, , ,0). In this study we compute the electronic (2, , ,0) and (2, , ) for water molecules in liquid water using QM/MM calculations. The average value of (2, , ,0) is smaller than the one for the gaz phase, and its standard deviation among the molecules is relatively small. In addition, we demonstrate that the average bulk second hyperpolarizability 〈(2, , ,0)〉 can be used to describe the electrostatic effects of the distant neighborhood on the first hyperpolarizability (2, , ). In comparison with more complex schemes to take into account long-range effects, the approximation is simple, and does not require any modifications of the QM/MM implementation. The long-range correction can be added explicitly, using an average value of for water in the condensed phase. It can also be easily added implicitly in QM/MM calculations through an additional embedding electric field, without the explicit calculation of .
分子的第一超极化率对从分子液体中收集的二阶光学非线性信号有贡献。对于二次谐波产生（SHG）响应，第一超极化率（β(2,ω,ω)）通常取决于分子的静电环境。对于水来说尤其如此，因为它具有较大的二阶超极化率（β(2,ω,ω,0)）。在本研究中，我们使用量子力学/分子力学（QM/MM）计算方法计算液态水中水分子的电子β(2,ω,ω,0)和β(2,ω,ω)。β(2,ω,ω,0)的平均值小于气相中的值，并且其在分子间的标准偏差相对较小。此外，我们证明平均体二阶超极化率〈β(2,ω,ω,0)〉可用于描述远处邻域对第一超极化率β(2,ω,ω)的静电效应。与考虑长程效应的更复杂方案相比，该近似方法简单且不需要对QM/MM实现进行任何修改。长程校正可以通过使用凝聚相中水的β值的平均值来明确添加。它也可以通过额外的嵌入电场在QM/MM计算中轻松隐式添加，而无需明确计算β。
