Abstract An adiabatic approach to compute quasibound states of rare gas–halogen van der Waals molecules is presented where the interhalogen vibrational coordinate is considered as the `slow' mode (i.e. it takes the place of the nuclear coordinates in the traditional Born–Oppenheimer separation). Although it might not seem intuitive (as far as the halogen vibrational motion is not slower than the intermolecular one), it is shown that a satisfactory (zero order) picture is provided for those states where the halogen subunit is near dissociation. The method is applied to HeBr2 (B,v), for which experimental data at very high v are available. Results are discussed in light of comparisons with `exact' close-coupling calculations.