Model space incompleteness in multireference state-universal and state-selective coupled-cluster theories

The role of the so-called C-conditions, originally introduced in the general model space (GMS), state-universal (SU), coupled-cluster (CC) theory with singles and doubles (GMS SU CCSD) to account for the internal cluster amplitudes that vanish in the case of a complete model space, is explored in the context of the state-selective Mukherjee MR-CC method (MkCCSD). Using three examples that involve a considerable quasidegeneracy we show that the C-conditions can be usefully employed also in the MkCCSD method once an incomplete (or general) model space is used.