Equation-of-motion/Linear-response Coupled Cluster Methods with an Approximate Treatment on Triples for Singly-excited States with Pronounced Double Excitation Character

In this work,we investigate the performance of various equation-of-motion/linear-response coupled cluster(EOM/LR-CC)methods with an approximate treatment for triples on excitation energies(EEs)by comparing with EOM-CCSDT(SDT=single,double,triple excitations)results.The focus of this work is on singly-excited states with percentages of the single excitation part(R_(1),%)from CC3 between 50%and 80%,i.e.,excited states with a pronounced double excitation character.CC3 is shown to provide EEs that agree well with EOM-CCSDT results for such excited states.Moreover,reliable EEs can be obtained with EOM-CCSD(T)(a)^(*) and CCSDR(3)for excited states with R1 from CC3 larger than 80%.As for singly-excited states with R1 from CC3 between 80%and 50%,EEs with EOM-CCSD^(*),CCSDR(T)andδ-CR-EOM-CC(2,3)-D agree reasonably well with those of EOM-CCSDT.However,it is too costly to choose a proper method for singly-excited states based on R_(1) of CC3 since CC3 is a rather expensive method.On the other hand,our results show that difference between EEs with EOM-CCSD and EOM-CCSD(T)(a)*[ΔE_((T)(a)*)]correlates well with R1 from CC3 andΔE_((T)(a)*)is about 0.25 eV when R_(1)(CC3)is 80%.Appropriate methods to obtain reasonable EEs for singly-excited state can be chosen based on whetherΔE_((T)(a)*)is larger than 0.25 eV.