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The chemisorption of CO on the atop site of Pt(111) has been simulated by a Pt4 cluster model. Ab initio self consistent field (SCF) and complete active space self consistent field (CASSCF) cluster model wave functions have been obtained for the electronic ground state. Likewise, ab initio SCF wavefunctions have been obtained for two other electronic states. The optimum geometry and vibrational frequencies of chemisorbed CO are reported for the three states. The interaction energy and vibrational shift of chemisorbed CO, with respect to free gas phase CO, have been analyzed for the three electronic states. This analysis is carried out by means of the constrained space orbital variation (CSOV) method. In all cases the bond is found to be dominated by σ donation and π back-donation, known as Blyholder's mechanism. This mechanism is further supported by SCF calculations on a larger, Pt13, cluster model. For both clusters, the CSOV analysis of the vibrational frequency definitely shows that, contrary to previous recent studies, a major contribution to the experimentally observed vibrational shift comes from the π back-donation mechanism. However, we found that, contrary to common belief, σ donation also acts to lower the CO frequency and not to increase it. Physical reasons for such unexpected behaviour are given.

The electrostatic potential distribution above the Pt(100), Pt(110) and Pt(111) surfaces has been studied using finite cluster model representations of the surfaces and their ab initio Hartree-Fock electron densities. From the different distributions it is seen that electrostatic effects are not substantial for Pt(100), minor but significant for Pt(110) and quite important for Pt(111). The minima of the electrostatic potential always appear at the more coordinated sites, four-fold for Pt(100) and Pt(110) and threefold for Pt(111), indicating that, up to first order, electrophilic adsorbates will prefer to approach the high coordinated surface sites.

An ab initio one-electron approach has been used to study the geometry and order of stabilities of Pt 3 and Pt 4 clusters. For the valence shell the Schrodinger equation is exactly solved in a rather large basis set, the effect of d electrons is modelled by a core polarization potential, and core—core corrections are also included. Results for Pt 3 are in good agreement with recent ab initio calculations which explicitly include the Pt 5d electrons. For Pt 4 the most stable structure is predicted to be a rhombus, as expected from comparison with other metal tetramers.