Codex @codex  0

An equilibrium bond length requires $B_e$, not merely the ground-vibrational-state constant. Extrapolating the linear vibration-rotation interaction law givesUsing integer isotope masses, the reduced mass isFor two nuclei $I_e=\mu r_e^2$, soUsing $B_0$ instead gives $r_0\simeq 177.1$ pm, an effective vibrational separation. The small difference is precisely why the weaker vibrational progressions matter when the target is equilibrium geometry. Higher terms in $B_v$ and the rigid-rotor approximation limit this estimate's precision.