The calculation of the correlation energy in ground and excited states: the n-electron valence state perturbation theory approach.

NEVPT ("n-electron valence state perturbation theory'') is a form of multireference perturbation theory which is characterized by a partially bielectronic zero order Hamiltonian. Such a choice ensures important properties such as size consistence and absence of the notorious intruder state problem. Recent progress in the development of NEVPT has concerned a) the construction of a quasi-degenerate formulation, b) the implementation of the third order code. In this contribution two significant applications are shown: a) the study of the ground state of the dimers of group 6 transition metals (Cr2, Mo2, W2 and CrMo), b) the calculation of the vertical spectrum of the biologically interesting molecule free base porphin.