Transformation of the Hamiltonian in excitation energy calculations: Comparison between Fock-space multireference coupled-cluster and equation-of-motion coupled-cluster methods

Abstract

A use of the transformed form of the Hamiltonian in excitation energy calculations is discussed and a comparison between methods using this form is performed. It is shown that use of a similarity-transformed Hamiltonian can lead to separation of the ground state and excited state eigenvalue problems. Then standard approaches for determination of excited state energies can be used. Since these energies are usually quasidegenerate, it is convenient to use the effective Hamiltonian scheme. For different approximate methods, various advantages and disadvantages referring to desirable features of a ‘‘theoretical model chemistry’’ are discussed.