Institut Charles Gerhardt (CTMM)-UMR 5253, CC 1501, Université Montpellier II-CNRS, 34095 Montpellier Cedex 05, France.
J Chem Phys. 2012 Jul 7;137(1):014305. doi: 10.1063/1.4722338.
We report the definition and testing of a new ab initio 12-dimensional potential for the water dimer with flexible monomers. Using our recent accurate CCpol-8s rigid water pair potential [W. Cencek, K. Szalewicz, C. Leforestier, R. van Harrevelt, and A. van der Avoird, Phys. Chem. Chem. Phys. 10, 4716 (2008)] as a reference for the undistorted monomers' geometries, a distortion correction has been added, which was taken from a former flexible-monomer ab initio potential. This correction allows us to retrieve the correct binding energy D(e)=21.0 kJ mol(-1), and leads to an equilibrium geometry in close agreement with the one obtained from benchmark calculations. The kinetic energy operator describing the flexible-monomer water dimer has been expressed in terms of Radau coordinates for each monomer and a recent general cluster polyspherical formulation describing their relative motions. Within this formulation, an adiabatic scheme has been invoked in order to decouple fast (intramolecular) modes and slow (intermolecular) ones. Different levels of approximation were tested, which differ in the way in which the residual potential coupling between the intramolecular modes located on different monomers and the dependence of the monomer rotational constants on the dimer geometry are handled. Accurate calculations of the vibration-rotation-tunneling levels of (H(2)O)(2) and (D(2)O)(2) were performed, which show the best agreement with experiments achieved so far for any water potential. Intramolecular excitations of the two monomers were calculated within two limiting cases, to account for the lack of non-adiabatic coupling between intramolecular modes due to the intermolecular motion. In the first model, the excitation was assumed to stay either on the donor or the acceptor molecule, and to hop between the two moieties upon donor-acceptor interchange. In the second model, the excitation remains on the same molecule whatever is the dimer geometry. Marginal frequency differences, less than 2 cm(-1), were obtained for all modes, and the resulting infrared shifts are in good agreement with experiments.
我们报告了一种新的从头算的 12 维水二聚体柔性单体势能的定义和测试。使用我们最近的精确 CCpol-8s 刚性水对势能[W. Cencek、K. Szalewicz、C. Leforestier、R. van Harrevelt 和 A. van der Avoird，Phys. Chem. Chem. Phys. 10，4716（2008）]作为未变形单体几何形状的参考，添加了一个变形校正，该校正来自以前的柔性单体从头算势能。该校正允许我们恢复正确的结合能 D(e)=21.0 kJ mol(-1)，并导致与从基准计算中获得的平衡几何形状非常接近。描述柔性单体水二聚体的动能算子已用每个单体的 Radau 坐标和最近描述它们相对运动的通用聚类多球公式表示。在这种表述中，引入了绝热方案以解耦快速（分子内）模式和慢速（分子间）模式。测试了不同的近似级别，它们在处理位于不同单体上的分子内模式之间的剩余势能耦合以及单体旋转常数对二聚体几何形状的依赖性方面有所不同。对（H(2)O)(2)和（D(2)O)(2)的振动-旋转-隧道能级进行了精确计算，与迄今为止任何水势能相比，这些能级与实验结果吻合得最好。在两种极限情况下计算了两个单体的分子内激发，以说明由于分子间运动而导致分子内模式之间缺乏非绝热耦合。在第一种模型中，激发被假定要么留在供体分子上，要么留在受体分子上，并且在供体-受体交换时在两个部分之间跳跃。在第二种模型中，无论二聚体几何形状如何，激发都保持在同一分子上。所有模式的边缘频率差异小于 2 cm(-1)，得到的红外位移与实验结果非常吻合。
