Coupled-cluster connected quadruples and quintuples corrections to the harmonic vibrational frequencies and equilibrium bond distances of HF, N(2), F(2), and CO.
Coupled-cluster connected quadruples and quintuples corrections to the harmonic vibrational frequencies and equilibrium bond distances of HF, N(2), F(2), and CO.
复制标题
对 HF、N(2)、F(2) 和 CO 的谐振频率和平衡键距进行耦合簇连接四元组和五元组校正。
作者：
T. Ruden;T. Helgaker;P. Jørgensen;J. Olsen
使用耦合簇方法，我们检查了连接的四重和五重虚拟激励对 HF、N(2)、F(2) 和 CO 的简谐振动频率和平衡键距的贡献。虽然四重对 N(2) 简谐频率的最大贡献为 -18.8 cm(-1)，对 F(2) 键距的最大五重贡献为 -3.9 pm cm(-1) 为 N(2) 的谐波频率，0.03 pm 为 N(2) 和 F(2) 的键距。当我们通过从带有扰动三元组校正的耦合簇单双元模型[CCSD(T)]改进描述到耦合簇单双元三元和四元组模型时，计算频率相对于实验的平均和最大绝对误差分别从11.3和15.8 cm(-1)减少到3.2和4.7 cm(-1)；对于键距，平均和最大绝对误差分别从 0.16 pm 和 0.47 pm 减少到 0.04 pm 和 0.13 pm。这里提出的计算证实了先前的观察，即在某种程度上，CCSD（T）平衡键距离和谐波频率中相对较小的误差是由于取消了连接三元组的近似（微扰）处理中的误差以及忽略了高阶连接激励而引起的。进一步纳入五元组贡献、相对论校正和绝热校正，将谐波频率的平均和最大绝对误差分别降低至 1.1 和 2.3 cm(-1)，键距分别降低至 0.02 和 0.05 pm。
Using the coupled-cluster method, we have examined the contributions from the connected quadruple and quintuple virtual excitations to the harmonic vibrational frequencies and equilibrium bond distances of HF, N(2), F(2), and CO. Whereas the largest quadruples contributions are -18.8 cm(-1) to the harmonic frequency of N(2) and 0.43 pm to the bond distance of F(2), the largest quintuples contributions are -3.9 cm(-1) to the harmonic frequency of N(2) and 0.03 pm to the bond distances of N(2) and F(2). As we improve the description by going from the coupled-cluster singles-and-doubles model with a perturbative triples correction [CCSD(T)] to the coupled-cluster singles-doubles-triples-and-quadruples model, the mean and maximum absolute errors in the calculated frequencies relative to experiment are reduced from 11.3 and 15.8 cm(-1), respectively, to 3.2 and 4.7 cm(-1); for the bond distances, the mean and maximum absolute errors are reduced from 0.16 and 0.47 pm, respectively, to 0.04 and 0.13 pm. The calculations presented here confirm previous observations that, to some extent, the relatively small errors in the CCSD(T) equilibrium bond distances and harmonic frequencies arise from a cancellation of errors in the approximate (perturbative) treatment of the connected triples and the neglect of higher-order connected excitations. Further inclusion of quintuples contributions, relativistic corrections, and adiabatic corrections reduces the mean and maximum absolute errors to 1.1 and 2.3 cm(-1), respectively, for the harmonic frequencies and to 0.02 and 0.05 pm, respectively, for the bond distances.