AbstractAn accurate knowledge of the long-range interaction potential for the ground and first few excited electronic states is needed for quantitative prediction of the rate coefficients for astrochemical reactions at low temperatures. Some reactions important for astrochemical modeling include an open-shell atom as one of the fragments. Due to the interplay between the spin-orbit and quadrupole interactions such reactions require a special treatment. In this paper we derive the general expressions for the energy levels for such systems, apply them to the C₂H(²Σ⁺)+O(³P) reaction, and compare the results with ab initio calculations.