Yang Jun, Hättig Christof
Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Universitatsstrasse 150 D-44801 Bochum, Germany.
J Chem Phys. 2009 Mar 28;130(12):124101. doi: 10.1063/1.3093947.
The equation-of-motion coupled-cluster method for excited states with the singles-and-doubles model (CCSD) has been implemented for ansatz 2 of the explicitly correlated CCSD(R12) and CCSD(F12) methods as part of the program package Dalton. In this model, an orthonormal complementary auxiliary basis set is used for the resolution-of-identity approximation in order to calculate the three-electron integrals needed for CCSD(R12) and CCSD(F12). The additional CCSD(R12) or CCSD(F12) terms introduced within ansatz 2, which are not present in ansatz 1, are derived and discussed with regard to the extra costs needed for their computation. As a first application the basis set convergence of equilibrium bond lengths and harmonic vibrational frequencies has been investigated for some singlet excited states of the diatomic molecules N(2), CO, BF, and BH. The calculated CCSD(F12) results show that the average absolute deviations of the bond lengths and frequencies from the basis set limits are below 0.1 pm and 5 cm(-1) as well as 0.05 pm and 1 cm(-1) for the triple- and quadruple-zeta basis sets, respectively. These deviations are shown to largely arise from the SCF basis set incompleteness errors.
作为程序包Dalton的一部分，已针对显式相关的CCSD(R12)和CCSD(F12)方法的假设2实现了具有单双激发态模型（CCSD）的运动方程耦合簇方法。在该模型中，使用正交互补辅助基组进行单位分解近似，以计算CCSD(R12)和CCSD(F12)所需的三电子积分。推导并讨论了假设2中引入的、假设1中不存在的额外CCSD(R12)或CCSD(F12)项及其计算所需的额外成本。作为首次应用，研究了双原子分子N₂、CO、BF和BH的一些单重激发态的平衡键长和简谐振动频率的基组收敛性。计算得到的CCSD(F12)结果表明，对于三重和四重ζ基组，键长和频率相对于基组极限的平均绝对偏差分别低于0.1 pm和5 cm⁻¹以及0.05 pm和1 cm⁻¹。结果表明，这些偏差主要源于自洽场基组不完备误差。
