本项目计划在CCSD级别上开发含旋轨耦合的Fock空间耦合簇理论(FSCCSD)计算方法和程序，以计算闭壳层重元素体系的激发能、双电离能和双电子亲和能。我们在求解耦合簇方程时才考虑旋轨耦合效应，通过运动方程耦合簇方法（EOM-CCSD）得到的电离态和电子亲和态波函数计算相应Fock空间的FSCC簇算符，采用Meissner提出的IHFSCC方法避免入侵态问题。由于单激发耦合簇算符能很好地描述旋轨耦合效应引起的轨道弛豫，与在自洽场计算中就考虑旋轨耦合的FSCC方法相比，我们所发展方法的计算量小还能保持较高的计算精度。与EOM-CCSD方法相比，FSCCSD的计算量更低而计算结果的精度相当。此外，我们还将探索不显著牺牲计算精度而且适用于FSCCSD方法的近似耦合簇算符计算方法，降低FSCCSD方法的计算量，使之能用于更大的重元素体系。

In this project, we propose to develop computational method and program for excitation energies, double ionization potentials and double electron affinities of closed-shell heavy element systems using Fock-space coupled-cluster theory at the singles and doubles level (FSCCSD) with spin-orbit coupling. Spin-orbit coupling will be included in solving the coupled-cluster (CC) equations and the FSCC cluster operator will be determined from wavefunctions in equation-of-motion coupled-cluster (EOM-CCSD) calculations for ionization potentials or electron affinities. The intermediate Hamiltonian FSCC approach proposed by Meissner will be adopted to avoid the intruder-state problem. Compared with the FSCC approach where SOC is included in SCF calculations, the present approach is able to afford reliable results at reduced computational cost since orbital relaxation due to SOC can be treated effectively via single excitation amplitudes. Furthermore, computational effort of FSCCSD is lower than that of EOM-CCSD, while results of these two approaches are of similar accuracy. In addition, approximate method for the CC cluster operator will also be explored to reduce computational effort of FSCC without sacrificing accuracy so that it can be applied to larger systems.