The semiempirical HFD model of intermolecular forces is extended to strongly anisotropic atom-diatom systems. A single-centre representation of the dispersion energy is found to be inadequate, and a method of partitioning dispersion forces between two centres of attraction is developed, based on the A tensor of the diatomic molecule. The method is tested on the systems Ar-HCl and Ar-HF, for which accurate data sensitive to the potential anisotropy are available, and gives promising results. The quality of the SCF potential used is crucial, and it appears that for Ar-HCl the results are limited by deficiencies in the SCF potentials available. For Ar-HF, the dispersion coefficients are not well known, and the HFD method provides a single-parameter functional form capable of reproducing all the available data.