ab initio Calculations
The best recent determinations of global potential energy surfaces have used ab initio surfaces as a staring point. With this in mind, and to account for the overall pattern of levels, calculations were performed with the MOLPRO package. Two Beowulf clusters of Intel Pentium dual CPU systems available within the School of Chemistry at Bristol were used. To obtain good quality overall potential energy surfaces a wide selection of points was calculated at the CI level including the lowest four symmetric and antisymmetric states in Cs symmetry. Calculating four states simultaneously was essential to give smooth curves, particularly for the A1Πu state; omitting even the highest state, the 1Πg state (see next page), which does not cross the A1Πu state gave unphysical kinks in the potentials.