The Xα valence bond theory of weak electronic coupling. Application to the low-lying states of Mo2Cl84−

We show that valence bond (VB) concepts can be introduced into Xα theory. The resulting Xα–VB model yields energy states which either are pure multiplets or can be combined by straightforward projection to give pure multiplets. The new theory should be more computationally efficient than Hartree–Fock-based CI models. A preliminary study of the δ→δ* transition in Mo2Cl84− yields an excitation energy closer to experiment than previous theoretical values, including those obtained to date from GVB–CI calculations.