Zhao Hewang, Wang Zhifan, Guo Minggang, Wang Fan
Institute of Atomic and Molecular Physics, Key Laboratory of High Energy Density Physics and Technology, Ministry of Education, Sichuan University, Chengdu, People's Republic of China.
School of Electronic Engineering, Chengdu Technological University, Chengdu 611730, China.
J Chem Phys. 2020 Apr 7;152(13):134105. doi: 10.1063/1.5145077.
Multireference methods are usually required for transition metal systems due to the partially filled d electrons. In this work, the single-reference equation-of-motion coupled-cluster method at the singles and doubles level for double ionization potentials (EOM-DIP-CCSD) is employed to calculate energies of states from the d configuration of late-transition metal atoms starting from a closed-shell reference. Its results are compared with those from the multireference Fock-space coupled-cluster method at the CCSD level (FSCCSD) for DIP from the same closed-shell reference. Both scalar-relativistic effects and spin-orbit coupling are considered in these calculations. Compared with all-electron FSCCSD results with four-component Dirac-Coulomb Hamiltonian, FSCCSD with relativistic effective core potentials can provide reasonable results, except for atoms with unstable reference. Excitation energies for states in the (n - 1)dns configuration are overestimated pronouncedly with these two methods, and this overestimation is more severe than those in the (n - 1)dns configuration. Error of EOM-CCSD on these excitation energies is generally larger than that of FSCCSD. On the other hand, relative energies of most of the states in the d configuration with respect to the lowest state in the same configuration are predicted reliably with EOM-DIP-CCSD, except for the P state of Hg and states in Ir. FSCCSD can provide reasonable relative energies for the several lowest states, while its error tends to be larger for higher states.
由于d电子部分填充，过渡金属体系通常需要多参考方法。在本工作中，采用用于双电离势的单参考运动方程耦合簇单双激发水平方法（EOM-DIP-CCSD），从闭壳层参考态出发计算晚期过渡金属原子d组态的态能量。将其结果与来自相同闭壳层参考态的双电离势的多参考福克空间耦合簇CCSD水平方法（FSCCSD）的结果进行比较。这些计算中考虑了标量相对论效应和自旋轨道耦合。与使用四分量狄拉克-库仑哈密顿量的全电子FSCCSD结果相比，具有相对论有效核势的FSCCSD可以提供合理的结果，但对于参考态不稳定的原子除外。这两种方法对(n - 1)dns组态中态的激发能都有明显高估，且这种高估比对(n - 1)dns组态中的更严重。EOM-CCSD对这些激发能的误差通常大于FSCCSD的误差。另一方面，除了Hg的P态和Ir中的态外，EOM-DIP-CCSD能可靠地预测d组态中大多数态相对于同一组态中最低态的相对能量。FSCCSD可以为几个最低态提供合理的相对能量，而对于较高态其误差往往更大。
