Hamm Peter
Department of Chemistry, University of Zurich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland.
J Chem Phys. 2014 Nov 14;141(18):184201. doi: 10.1063/1.4901216.
In a recent paper, the experimental 2D-Raman-THz response of liquid water at ambient conditions has been presented [J. Savolainen, S. Ahmed, and P. Hamm, Proc. Natl. Acad. Sci. U. S. A. 110, 20402 (2013)]. Here, all-atom molecular dynamics simulations are performed with the goal to reproduce the experimental results. To that end, the molecular response functions are calculated in a first step, and are then convoluted with the laser pulses in order to enable a direct comparison with the experimental results. The molecular dynamics simulation are performed with several different water models: TIP4P/2005, SWM4-NDP, and TL4P. As polarizability is essential to describe the 2D-Raman-THz response, the TIP4P/2005 water molecules are amended with either an isotropic or a anisotropic polarizability a posteriori after the molecular dynamics simulation. In contrast, SWM4-NDP and TL4P are intrinsically polarizable, and hence the 2D-Raman-THz response can be calculated in a self-consistent way, using the same force field as during the molecular dynamics simulation. It is found that the 2D-Raman-THz response depends extremely sensitively on details of the water model, and in particular on details of the description of polarizability. Despite the limited time resolution of the experiment, it could easily distinguish between various water models. Albeit not perfect, the overall best agreement with the experimental data is obtained for the TL4P water model.
在最近的一篇论文中，已经给出了液态水在环境条件下的实验二维拉曼 - 太赫兹响应[J. 萨沃莱宁、S. 艾哈迈德和P. 哈姆，《美国国家科学院院刊》110, 20402 (2013)]。在此，进行全原子分子动力学模拟的目的是重现实验结果。为此，第一步计算分子响应函数，然后将其与激光脉冲进行卷积，以便能够与实验结果进行直接比较。分子动力学模拟使用了几种不同的水模型：TIP4P/2005、SWM4 - NDP和TL4P。由于极化率对于描述二维拉曼 - 太赫兹响应至关重要，在分子动力学模拟之后，对TIP4P/2005水分子后验地修正了各向同性或各向异性极化率。相比之下，SWM4 - NDP和TL4P本质上是可极化的，因此可以使用与分子动力学模拟期间相同的力场，以自洽的方式计算二维拉曼 - 太赫兹响应。研究发现，二维拉曼 - 太赫兹响应极其敏感地依赖于水模型的细节，特别是极化率描述的细节。尽管实验的时间分辨率有限，但它能够轻松区分各种水模型。尽管并不完美，但对于TL4P水模型，总体上与实验数据的一致性最佳。
