Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 16628 Prague 6, Czech Republic.
J Chem Phys. 2011 Oct 21;135(15):154301. doi: 10.1063/1.3649942.
We have performed large-scale simulations of UV absorption spectra of water clusters (monomer to octamer) using a combination of ab initio path-integral molecular dynamics with reflection principle. The aim of the present work is four-fold: (1) To explore the transition from isolated molecules to bulk water from the perspective of UV photoabsorption. (2) To investigate quantum nuclear and thermal effects on the shape of the water UV spectra. (3) To make an assessment of the density functional theory functionals to be used for water excited states. (4) To check the applicability of the QM/MM schemes for a description of the UV absorption. Within the path integral molecular dynamics (PIMD)/reflection principle approach both the thermal and quantum vibrational effects including anharmonicities are accounted for. We demonstrate that shape of the spectra is primarily controlled by the nuclear quantum effects. The excited states and transition characteristics of the water clusters were calculated with the time-dependent density functional theory and equation-of-motion coupled clusters singles and doubles methods. Based on our benchmark calculations considering the whole UV spectrum we argue that the BHandHLYP method performs best among the 6 functionals tested (B3LYP, BHandHLYP, BNL, CAM-B3LYP, LC-ωPBE, and M06HF). We observe a gradual blueshift of the maximum of the first absorption peak with the increasing cluster size. The UV absorption spectrum for the finite size clusters (i.e., the peak centers, peak widths, and photoabsorption cross section) essentially converges into the corresponding bulk water spectrum. The effect of distant molecules accounted for within the polarizable continuum model is shown to be almost negligible. Using the natural transition orbitals we demonstrate that the first absorption band is formed by localized excitations while the second band includes delocalized excited states. Consequently, the QM/MM electrostatic embedding scheme can only be used for the modeling of the low energy part of the spectrum.
我们使用从头算路径积分分子动力学与反射原理相结合的方法，对水分子簇（单体到八聚体）的紫外吸收光谱进行了大规模的模拟。本工作有四个目的：（1）从紫外光吸收的角度探讨从孤立分子到体相水的转变。（2）研究量子核和热效应对水紫外光谱形状的影响。（3）评估用于水激发态的密度泛函理论泛函。（4）检验 QM/MM 方案用于描述紫外吸收的适用性。在路径积分分子动力学（PIMD）/反射原理方法中，同时考虑了热和量子振动效应，包括非谐性。我们证明了光谱的形状主要由核量子效应控制。用含时密度泛函理论和方程运动耦合簇单双激发方法计算了水分子簇的激发态和跃迁特性。基于我们考虑整个紫外光谱的基准计算，我们认为在测试的 6 种泛函（B3LYP、BHandHLYP、BNL、CAM-B3LYP、LC-ωPBE 和 M06HF）中，BHandHLYP 方法表现最好。我们观察到随着团簇尺寸的增加，第一个吸收峰的最大值逐渐蓝移。有限尺寸团簇的紫外吸收光谱（即峰中心、峰宽和光吸收截面）基本上收敛到相应的体相水光谱。在极化连续模型中考虑到的远距离分子的影响几乎可以忽略不计。通过自然跃迁轨道，我们证明第一个吸收带是由局域激发形成的，而第二个带包含离域激发态。因此，QM/MM 静电嵌入方案只能用于光谱的低能部分的建模。
