高次激发相关能在构建HCN—Here光谱精度势能面中的角色：通过实验高分辨率光谱进行严格检测

The Role of High Excitations in Constructing Sub-spectroscopic Accuracy Intermolecular Potential of He-HCN： Critically Examined by the High-Resolution Spectra with Resonance States

解释弱相互作用体系的高分辨率振转光谱通常需要光谱精度高（误差小于1个波数）的势能面．构建高精度的从头算势能面依赖于高水平的电子结构计算方法和准确的物理模型来表示势能面．而对于电子结构计算，包括单双激发和微扰处理三重激发的耦合簇的方法CCSD（T）被誉为“黄金标准”，被广泛的用于计算范德华体系的分子间势能面上．然而，对于HCN—He体系，即使采用CCSD（T）方法和完备基组（CBS）计算势能面，其精度也达不到指认、解释该体系观测到的含有高激振转态的微波光谱．本文在CCSD（T1／CBS水平上计算了包括HCN分子C—H（Q1）振动坐标的三维分子间势能面，并且通过CCSDT（Q）方法计算了高次激发相关能．结果表明，含有CCSDT（Q）高次激发修正能的势能面，其预测的能级和跃迁频率与实验数据都非常吻合，均方根偏差达到亚光谱精度（仅0．072cm-1），而不包括该校正能，其误差将会扩大到5—7倍，清楚地说明了高次激发相关在构建亚光谱精度势能面中的角色和重要意义．此外，本文首次预测了HCN—He体系的高分辨红外光谱．

Interpreting high-resolution rovibrational spectra of weakly bound complexes commonly requires spectroscopic accuracy （〈1 cm-1） potential energy surfaces （PES）. Constructing high-accuracy ab initio PES relies on the high-level electronic structure approaches and the accurate physical models to represent the potentials. The coupled cluster approaches including single and double excitations with a perturbational estimate of triple excitations （CCSD（T）） have been termed the ＂gold standard＂ of electronic structure theory, and widely used in generating intermolecular interaction energies for most van der Waals complexes. However, for HCN-He complex, the observed millimeter-wave spectroscopy with high-excited resonance states has not been assigned and interpreted even on the ab initio PES computed at CCSD（T） level of theory with the complete basis set （CBS） limit. In this work, an effective three-dimensional ab initio PES for HCN-He, which explicitly incorporates dependence on the Q1 （C-H） normal-mode coordinate of the HCN monomer has been calculated at the CCSD（T）/CBS level. The post-CCSD（T） interaction energy has been examined and included in our PES. Analytic two-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for v1 （C-H）=0, and 1 to the Morse/Long-Range potential function form with root-mean-square deviations （RMSD） smaller than 0.011 cm-1. The role and significance of the post-CCSD（T） interaction energy contribution are clearly illustrated by comparison with the predicted rovibrational energy levels. With or without post-CCSD（T） corrections, the value of dissociation limit （Do） is 8.919 or 9.403 cm-1, respectively. The predicted millimeter-wave transitions and intensities from the PES with post-CCSD（T） excitation corrections are in good agreement with the available experimental data with RMS discrepancy of 0.072 cm-1. Moreover, the infrared spectrum for HCN-He complex is predicted for the first time. These results will serve as a good starting point and provide reliable guidance for future infrared studies of HCN doped in （He）n clusters.