  We investigate Born-Oppenheimer breakdown (BOB) effects (beyond the usual
mass scaling) for the electronic ground states of a series of homonuclear and
heteronuclear alkali-metal diatoms, together with the Sr$_2$ and Yb$_2$
diatomics. Several widely available electronic structure software packages are
used to calculate the leading contributions to the total isotope shift for
commonly occurring isotopologs of each species. Computed quantities include
diagonal Born-Oppenheimer corrections (mass shifts) and isotopic field shifts.
Mass shifts dominate for light nuclei up to and including K, but field shifts
contribute significantly for Rb and Sr and are dominant for Yb. We compare the
{\em ab initio} mass-shift functions for Li$_2$, LiK and LiRb with
spectroscopically derived ground-state BOB functions from the literature. We
find good agreement in the values of the functions for LiK and LiRb at their
equilibrium geometries, but significant disagreement with the shapes of the
functions for all 3 systems. The differences may be due to contributions of
nonadiabatic terms to the empirical BOB functions. We present a semiclassical
model for the effect of BOB corrections on the binding energies of
near-threshold states and the positions of zero-energy Feshbach resonances.
