The GIPM/D (ground state inversion potential method/diffraction) theory recently described is applied to the prediction and interpretation of photoionisation intensities of F 2, Cl 2 and HCl. The theory accounts for atomic one-electron effects with a local approximation for exchange and takes the continuum energy eigenfunctions of the molecule to be locally atomic, thereby allowing interpretation in terms of the concepts of diffraction theory. Total subshell cross sections and angular asymmetry factors have been calculated in modest basis sets (usually double zeta quality) for all valence and some inner valence molecular orbitals over a range of photon energies from threshold to 100 eV. Extensive comparison both with measurement and with more detailed theory shows that the simple diffraction approach can provide qualitative and semiquantitative explanations for most of the magnitudes and energy dependencies seen in experiment. The concept of interference enhancement is shown to be valuable in relating cross sections/energy relationships to orbital type.