Accurate ab initio potential energy curve of O2. II. Core-valence correlations, relativistic contributions, and vibration-rotation spectrum

In the first paper of this series, a very accurate ab initio potential energy curve of the (3)Sigma(-)(g) ground state of O-2 has been determined in the approximation that all valence shell electron correlations were calculated at the complete basis set limit. In the present study, the corrections arising from core electron correlations and relativity effects, viz., spin-orbit coupling and scalar relativity, are determined and added to the potential energy curve. From the 24 points calculated on this curve, an analytical expression in terms of even-tempered Gaussian functions is determined and, from it, the vibrational and rotational energy levels are calculated by means of the discrete variable representation. We find 42 vibrational levels. Experimental data (from the Schumann-Runge band system) only yield the lowest 36 levels due to significant reduction in the transition intensities of higher levels. For the 35 term values G(v), the mean absolute deviation between theoretical and experimental data is 12.8 cm(-1). The dissociation energy with respect to the lowest vibrational energy is calculated within 25 cm(-1) of the experimental value of 41 268.2 +/- 3 cm(-1). The theoretical crossing between the (3)Sigma(-)(g) state and the (1)Sigma(+)(g) state is found to occur at 2.22 A degrees and the spin-orbit coupling in this region is analyzed.