Spin-orbit interaction in the HD${}^{+}$ ion

We report on progress in calculation of the spin-orbit interaction for the HD${}^{+}$ molecular ion. This interaction is currently the largest source of theoretical uncertainty in determination of the hyperfine structure of rovibrational transition lines. The corrections of order $m{\alpha }^7\ln (\alpha )$ are derived and numerically calculated. Theoretical hyperfine intervals are compared with experimental data, and the observed discrepancies are discussed.