Third-order multireference perturbation theory:: The n-electron valence state perturbation-theory approach -: art. no. 054108

A formulation of the n-electron valence state perturbation theory (NEVPT) at the third order of perturbation is presented. The present implementation concerns the so-called strongly contracted variant of NEVPT, where only a subspace of the first-order interacting space is taken into account. The resulting strongly contracted NEVPT3 approach is discussed in three test cases: (a) the energy difference between the B-3(1) and (1)A(1) states of the methylene molecule, (b) the potential-energy curve of the N-2 molecule ground state, and (c) the chromium dimer (Cr-2) ground-state potential-energy profile. Particular attention is devoted to the last case where large basis sets comprising also h orbitals are adopted and where remarkable differences between the second- and third-order results show up. (c) 2006 American Institute of Physics.