  The low-energy electron-impact dissociation of molecular hydrogen has been a
source of disagreement between various calculations and measurements for
decades. Excitation of the ground state of H$_2$ into the dissociative $b
^3\Sigma_u^+$ state is now well understood, with the most recent measurements
being in excellent agreement with the molecular convergent close-coupling
(MCCC) calculations of both integral and differential cross sections (2018
Phys. Rev. A 98 062704). However, in the absence of similar measurements for
vibrationally-excited or isotopically-substituted H$_2$, cross sections for
dissociation of these species must be determined by theory alone. We have
identified large discrepancies between MCCC calculations and the recommended
$R$-matrix cross sections for dissociation of vibrationally-excited H$_2$,
D$_2$, T$_2$, HD, HT, and DT (2002 Plasma Phys. Contr. F. 44
1263-1276,2217-2230), with disagreement in both the isotope effect and
dependence on initial vibrational level. Here we investigate the source of the
discrepancies, and discuss the consequences for plasma models which have
incorporated the previously recommended data.
