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We present a novel and cost-effective approach of using a second similaritytransformation of the Hamiltonian to include the missing higher-order terms inthe second-order approximate coupled cluster singles and doubles (CC2) model.The performance of the newly developed ST-CC2 model has been investigated forthe calculation of excitation energies of valence, Rydberg, and charge-transferexcited states. The method shows significant improvement in the excitationenergies of Rydberg and charge-transfer excited states as compared to theconventional CC2 method while retaining the good performance of the latter forthe valence excited state. The method retains the charge-transfer separabilityof the CT excited states, which is a significant advantage over the traditionalCC2 method. An MBPT2 variant of the new method is also proposed.

A Similarity Transformed Second-order Approximate Coupled Cluster Method for the Excited States: Theory, Implementation, and Benchmark