Noniterative coupled-cluster methods for excited electronic states

New classes of noniterative coupled-cluster (CC) methods, which improve the results of the standard equation-of-motion (EOM) and response CC calculations for excited states dominated by two-electron transitions and excited-state potential energy surfaces along bond breaking coordinates, are reviewed. All of the methods discussed in this article are derived from the method of moments of CC equations (MMCC) and all of them are characterized by the relatively low computer costs which are similar to those characterizing the popular ground-state CCSD(T) theory. Three types of approaches are discussed: (i) the externally corrected MMCC approaches employing the configuration interaction and multi-reference perturbation theory wave functions, (ii) the completely renormalized EOMCC methods, including their most recent extension to excited states of radicals and other open-shell systems, and (iii) the new classes of MMCC and completely renormalized EOMCC theories employing the left eigenstates of the similarity-transformed Hamiltonian used in CC/EOMCC theory.