Multireference coupled-cluster approaches to excited states

We review our recent work on multireference (MR) coupled-cluster (CC) methodology, namely (i) the idea of external corrections that are essential for the so-called reduced MR (RMR) CCSD and RMR-CCSD(T) methods at the single-reference (SR) level and for (N,M)-CCSD at the MR level, and (ii) the concept of the so-called C-conditions for the internal cluster amplitudes at the genuine MR CC state universal (SU) level. The latter concept enables the use of general model spaces (GMSs) while preserving the intermediate normalization and can be employed in any MR CC method that is based on the SU cluster Ansatz of Jeziorski and Monkhorst, including state-selective (SS) approaches, such as the Mukherjee MkCCSD and Brillouin-Wigner BW-CC methods. The performance of the RMR-CCSD(T), GMS-SU-CCSD, GMS-MkCCSD, and (N,M)-CCSD approaches is illustrated on a few typical examples.