Calibration of the n-electron valence state perturbation theory approach.

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X2 (X=N, P, As, and Sb) and the 4S/2D and 4S/2P splittings for the corresponding atoms, the 1A1–3B1 splittings for CH2 and SiH2, and the absorption spectra of pyrrole and of Cu(Imidazole)2(SH)(SH2)+, which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the 4S/2D and 4S/2P splittings for the group fifteen atoms. This is due to the different treatment of orbita...