Metz Michael P, Szalewicz Krzysztof, Sarka János, Tóbiás Roland, Császár Attila G, Mátyus Edit
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA.
Phys Chem Chem Phys. 2019 Jun 26;21(25):13504-13525. doi: 10.1039/c9cp00993k.
Motivated by the energetic and environmental relevance of methane clathrates, highly accurate ab initio potential energy surfaces (PESs) have been developed for the three possible dimers of the methane and water molecules: (H2O)2, CH4·H2O, and (CH4)2. While only a single monomer geometry was used for each monomer in the ab initio calculations, the PES parameterization makes it possible to produce distinct surfaces for all isotopologues within the rigid-monomer approximation. The PESs were fitted to computations at the frozen-core coupled-cluster level with single, double, and non-iterative triple excitations, employing basis sets of augmented triple- and quadruple-zeta quality plus bond functions, followed by extrapolations to the complete basis set limit. The long-range parts of the PESs are computed using the asymptotic version of symmetry-adapted perturbation theory based on a density-functional description of the monomers. All PESs are polarizable, i.e., in cluster or condensed-phase applications they approximate many-body effects by the induced dipole polarization model. The PESs were developed in a fully automated procedure applying the autoPES method, which is used for the first time to generate near-spectroscopic quality surfaces. The stationary points (SPs) on the PESs have been determined and compared with literature data. For CH4·H2O, previously unknown SPs have been identified and the first detailed study of the (CH4)2 potential energy landscape has been carried out. The PESs were used in variational quantum nuclear motion computations. For the water dimer, the resulting vibrational transitions are in excellent agreement with available high-resolution spectroscopic data. For (CH4)2, the intermonomer vibrational states are reported for the first time.
受甲烷笼合物能量和环境相关性的驱动，针对甲烷和水分子的三种可能二聚体：(H2O)2、CH4·H2O和(CH4)2，已开发出高精度的从头算势能面（PES）。虽然在从头算计算中每个单体仅使用了单一的单体几何结构，但PES参数化使得在刚性单体近似下能够为所有同位素异构体生成不同的表面。这些PES通过在冻结核心耦合簇水平上进行计算拟合，包括单、双和非迭代三激发，采用增强的三重和四重ζ质量基组加上键函数，随后外推到完全基组极限。PES的长程部分使用基于单体密度泛函描述的对称适应微扰理论的渐近版本进行计算。所有PES都是可极化的，即在团簇或凝聚相应用中，它们通过诱导偶极极化模型近似多体效应。这些PES是通过应用autoPES方法的全自动程序开发的，该方法首次用于生成接近光谱质量的表面。已确定了PES上的驻点（SP）并与文献数据进行了比较。对于CH4·H2O，已识别出先前未知的SP，并首次对(CH4)2势能面进行了详细研究。这些PES用于变分量子核运动计算。对于水二聚体，所得的振动跃迁与现有的高分辨率光谱数据非常吻合。对于(CH4)2，首次报道了单体间的振动态。
