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

I n 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 R 1 from CC3 larger than 80%. As for singly-excited states with R 1 from CC3 between 80% and 50%, EEs with EOM-CCSD*, CCSDR(T) and δ -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)* [Δ E (T)(a)* ] correlates well with R 1 from CC3 and Δ E (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 whether Δ E (T)(a)* is larger than 0.25 eV.