Structures and harmonic vibrational frequencies for excited statesof diatomic molecules with CCSD(R12) and CCSD(F12) models

摘要

The equation-of-motion coupled-cluster method for excited states with the singles-and-doublesmodel (CCSD) has been implemented for ansatz 2 of the explicitly correlated CCSD(R12) andCCSD(F12) methods as part of the program package Dalton. In this model, an orthonormalcomplementary auxiliary basis set is used for the resolution-of-identity approximation in order tocalculate the three-electron integrals needed for CCSD(R12) and CCSD(F12). The additionalCCSD(R12) or CCSD(F12) terms introduced within ansatz 2, which are not present in ansatz 1, arederived and discussed with regard to the extra costs needed for their computation. As a firstapplication the basis set convergence of equilibrium bond lengths and harmonic vibrationalfrequencies has been investigated for some singlet excited states of the diatomic molecules N2, CO,BF, and BH. The calculated CCSD(F12) results show that the average absolute deviations of thebond lengths and frequencies from the basis set limits are below 0.1 pm and 5 cm~(-1)as well 0.05pm and 1 cm~(-1)for the triple- and quadruple-ζ basis sets, respectively. These deviations are shownto largely arise from the SCF basis set incompleteness errors.