A new method for excited states: Similarity transformed equation-of-motion coupled-cluster theory

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Abstract
We present the first application of the similarity transformed equation-of-motion coupled-cluster method (STEOM-CC) to calculate excited states. STEOM-CC theory arises from a similarity transform of the second quantized Hamiltonian which strongly reduces the coupling between singly excited determinants and more highly excited configurations. Consequently, excitation energies can be obtained to a good approximation by diagonalizing the transformed Hamiltonian in the space of single excitations only. The STEOM method is applied to obtain the valence excitation spectrum of the pyridine molecule. The accuracy of STEOM is shown to be comparable to current state of the art methods like equation-of-motion coupled-cluster theory and CASPT2, whereas the computational requirements of STEOM are very modest compared to the above methods.