Knowledge Bank

For diatomic molecules $Re$, the equilibrium internuclear distance, is conventionally determined from spectroscopic data by least square fitting the $Bv$ values to $$B_{v}=B_{e}-\alpha_{e}(v+1/2)+\gamma_{e}(v+1/2)^{2}\cdots$$ giving $$ R_{e}=(\hbar/4\Pi c \mu B_{e})^{1/2} $$ where $\mu$ is the reduced mass. For some potential curves this does not yield $Re$ as the potential energy minimum. For example in the $A1\Sigma +$ state of LiH the discrepancy is about 0.1 \AA. This problem has been analyzed using an extensive Dunham analysis and model potentials, and corrections are suggested.