Intruders multireference methods

The data of Table 3b have been computed by replacing the active space of the CASPT2 treatment by the CIPSI multireference space of the MRPT2 method documented in section 2.1. The selection of the components of the reference space by this technique enables us to reduce the gap between the Moller-Plesset and Epstein-Nesbet total energies to less than 0.01 a.u., and so to improve our evaluation of energy balances, disregarding a possible overestimation of the Moller-Plesset values due to intruder states, as it is the case in the ScNC system. 

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