Multireference composite approaches for the accurate study of ground and excited electronic states: C2, N2, and O2

A multireference analog of the correlation consistent composite approach (MR-ccCA) based on complete active space with second-order perturbation theory (CASPT2) has been utilized in an investigation of the ground and valence excited states of C-2, N-2, and O-2. The performance of different second-order multireference perturbation theory methods including second-order n-electron valence state perturbation theory, second-order multireference Moller-Plesset, and second-order generalized van Vleck perturbation theory has been analyzed as potential alternatives to CASPT2 within MR-ccCA. The MR-ccCA-P predicts spectroscopic constants with overall mean absolute deviations from experimental values of 0.0006 angstrom, 7.0 cm(-1), and 143 cm(-1) for equilibrium bond length (r(e)), harmonic frequency (omega(e)), and term values (T-e), respectively, which are comparable to the predictions by more computationally costly multireference configuration interaction-based methods. (C) 2011 American Institute of Physics. [doi:10.1063/1.3514031]