### 7. Equilibrium Internuclear Distances

When precise values for the Dunham coefficients, *Y*_{lj}, are
obtained from experiment, the equilibrium internuclear distance can be
determined from the equilibrium rotational constant *B*_{e}.
Dunham [3] derived the following relationship between
*Y*_{01} and *B*_{e}:

where the *a*_{i} are the anharmonic potential constants that can
be evaluated from the higher *Y*_{ij} coefficients. The
equilibrium internuclear distance is then expressed as:

Since Dunham's treatment utilizes the Born-Oppeniheimer approximation, the
value of *B*_{e} must be modified for breakdown of this
approximation. Several methods have been developed for handling these
corrections; the most important are those of Rosenblum et al.
[58008],
Bunker [17], and Watson
[73023].