THE ELECTRONIC STRUCTURE OF THE DIATOMIC MOLECULE SCANDIUM-LITHIUM

The lowest states of Sc-Li which correlate with $Li (^{2}S)$ and either $Sc(^{2}D$; $4S^{2}3d^{1})$ or $Sc (^{4}F$; $4s3d^{2})$ have been studied using MCSCF+CI techniques. MCSCF calculations which correlate the $4s^{2}$ pair (near degeneracy) and allow for proper dissociation predict a repulsive interaction between the $Sc (^{2}D)$ and $Li(^{1}S)$ states. In contrast, similar level calculations on selected states $(^{3}\Sigma^{-}$ and $^{3}\phi)$ which correlate with $Sc(^{4}F)$ and $Li(^{2}S)$ predict a strongly bound (relative ot $Sc(^{4}F)$) molecule. These results suggest that as $Li(^{2}s)$ approaches the $Sc 4s^{2}3d^{1}$ configuration it is repelled by the $4s^{2}$ pair before it can bond to the 3d orbital whereas in the $4s3d^{2}$ configuration the 4s is easily available for bonding. CI calculations in which the four valence electrons are active do not change this qualitative interpretation