The three lowest adiabatic potential energy curves for each of the two dipole-allowed symmetries, (1)Sigma (+)(u) and (1)Pi (u), are calculated in the multireference configuration-interaction framework. Diabatic potentials and corresponding coupling elements are obtained by diagonalizing the electronic operator r(2) which serves to discriminate Rydberg and valence type states. A large basis set and judiciously chosen active orbital and configuration spaces furnish smooth and reliable potential curves. However, a vibrational analysis of the coupled systems in diabatic representation still shows some disappointing deviations from the experimental interference patterns of overlapping absorption bands that are highly sensitive to potential energy differences. Starting from the calculated curves, a fitting procedure accounting also for empirical information yields potential energy curves and diabatic coupling elements that reproduce all details of the experiment very well. These recommended results also serve to identify residual defects in the cab initio curves mainly as vertical shifts. The performance of other commonly used ab initio methods for the calculation of excited states is briefly discussed.