A quasidegenerate formulation of the second order n-electron valence state perturbation theory approach

Celestino Angeli; Stefano Borini; Mirko Cestari; Renzo Cimiraglia

doi:10.1063/1.1778711

A quasidegenerate formulation of the second order n-electron valence state perturbation theory approach

J Chem Phys. 2004 Sep 1;121(9):4043-9. doi: 10.1063/1.1778711.

Authors

Celestino Angeli¹, Stefano Borini, Mirko Cestari, Renzo Cimiraglia

Affiliation

¹ Dipartimento di Chimica, Universita di Ferrara, Via Borsari 46, I-44100 Ferrara, Italy.

PMID: 15332949
DOI: 10.1063/1.1778711

Abstract

The n-electron valence state perturbation theory (NEVPT) is reformulated in a quasidegenerate (QD) approach. The new theory allows the treatment of cases where the proximity of the energies causes artifacts in the zero order description. Problems of quasidegeneration are relevant in the dynamics involving regions at avoided crossings (or conical intersections) and in spectroscopy where the energies and oscillator strengths can be strongly influenced by the mixing of states of different nature. Two test cases are analyzed concerning (a) the ionic-neutral avoided crossing in LiF and (b) the valence/Rydberg mixing in the excited states of ethene. The QD-NEVPT2 is shown to be a useful tool for such systems.