A NEW METHOD FOR EXCITED-STATES - SIMILARITY TRANSFORMED EQUATION-OF-MOTION COUPLED-CLUSTER THEORY

We present the first application of the similarity transformed equatio n-of-motion coupled-cluster method (STEOM-CC) to calculate excited sta tes. STEOM-CC theory arises from a similarity transform of the second quantized Hamiltonian which strongly reduces the coupling between sing ly excited determinants and more highly excited configurations. Conseq uently, excitation energies can be obtained to a good approximation by diagonalizing the transformed Hamiltonian in the space of single exci tations only. The STEOM method is applied to obtain the valence excita tion 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 r equirements of STEOM are very modest compared to the above methods. (C ) 1997 American Institute of Physics.