title: Internally contracted multireference coupled-cluster theory in a multistate framework.
abstract: The internally contracted multireference coupled cluster (icMRCC) theory is reexamined in a multistate framework. The new derivation starts from the Bloch equations employing a wave operator similar to the one originally employed by Jeziorski and Monkhorst [Phys. Rev. A 24, 1668 (1981)], but allows for a multideterminantal model-space basis. The resulting working equations are a generalization of state-universal multireference coupled-cluster (SU-MRCC) theory. In the case of truncated cluster operators, energies and wave functions depend on the choice of the model-space basis. It is shown that the state-specific solutions of the original icMRCC theory exactly diagonalize the effective Hamiltonian of the multistate theory and thus decouple the multistate equations. While this in principle means that icMRCC is a multistate theory, we find that truncated icMRCC theory still shows minor artifacts close to avoided crossings, which can be removed by considering a true multistate ansatz. The results also shed new light on the orbital variance of the original SU-MRCC theory.
authors.affiliation: Institut für Theoretische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
authors.forename: Yuri Alexandre
authors.initials: YA
authors.lastname: Aoto
authors.affiliation: Institut für Theoretische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Köhn