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