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J Chem Phys. 2006 Feb 7;124(5):054108.

Third-order multireference perturbation theory: the n-electron valence state perturbation-theory approach.

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Dipartimento di Chimica, Università di Ferrara, Via Borsari 46, I-44100 Ferrara, Italy.


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 3B1 and 1A1 states of the methylene molecule, (b) the potential-energy curve of the N2 molecule ground state, and (c) the chromium dimer (Cr2) 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.


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