O2(a-X) splitting

[Roy Jensen <royj' at \`uvic.ca>]

All
 I was looking at the splitting between the a-1-delta and X-3-sigma states
 of oxygen using the UQCISD/6-311++G(d,p) level of theory. The calculated
 singlet-triplet spacing is 47 % higher than the experimentally observed
 value (11618 vs 7918 cm-1, respectively). I am sure there is a ready
 explanation for this; however, I am drawing a blank...any insights
 would be greatly appreciated!
 Roy Jensen
 -----BEGIN PGP PUBLIC KEY BLOCK-----
 Comment: available at www.consol.ca
 ------END PGP PUBLIC KEY BLOCK------