This paper will review the use of ab initio Hartree–Fock wave functions for small metal/ligand clusters as a basis for the interpretation of processes related to molecular chemisorption. The primary concern will be with absorbate derived photoemission spectra (PES). It will be shown that quite small clusters can, indeed, give considerable insight into the structure, shifts, and character of this spectra. Ab initio molecular orbital theory is a particularly valuable approach since it permits one to relate these features of the PES to the nature of the adsorbate bonding in a direct way. The satellite structure originating from screening of the absorbate hole by metal electrons will be analysed for both core and valence levels. The decomposition of adsorbate shifts into initial and final state contributions will be considered (in order to understand the relationship of these shifts to the binding).