Multireference equation-of-motion coupled cluster theory

A generalization of the equation-of-motion coupled cluster theory is proposed, which is built upon a multireference parent state. This method is suitable for a number of electronic states of a system that can be described by similar active spaces, i.e., different linear combinations of the same set of active space determinants. One of the suitable states is chosen as the parent state and the dominant dynamical correlation is optimized for this state using an internally contracted multireference coupled cluster ansatz. The remaining correlation and orbital relaxation effects are obtained via an uncontracted diagonalization of the transformed Hamiltonian, <(<(H)over bar>)over cap> = e(-(T) over cap) (H) over cap e((T) over cap), in a compact multireference configuration interaction space, which involves configurations with at most single virtual orbital substitution. The latter effects are thus state-specific and this allows us to obtain multiple electronic states in the spirit of the equation-of-motion coupled cluster approach. A crucial aspect of this formulation is the use of the amplitudes of the generalized normal-ordered transformed Hamiltonian <(<(H)over bar>)over cap> as the residual equations for determining the internally contracted cluster amplitudes without any projection onto the excited configurations. These residuals have been termed as the many-body residuals. These equations are formally non-singular and thus allow us to solve for all amplitudes without discarding any, in contrast to other internally contracted approaches. This is desirable to ensure transferability of dynamical correlation from the parent state to the target states. Preliminary results involving the low-lying electronic states of C-2, O-2, and the excitation spectra of three transition metal atoms, e. g., Fe, Cr, and Mn, including hundreds of excited states, illustrate the potential of our approach. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766361]