Similarities and differences in the Hartree-Fock and density-functional description of the chemisorption bond

The similarities and differences between the ab initio Hartree-Fock (HF) an d density-functional theory (DFT) descriptions of the chemisorption bond ha ve been explored by applying the constrained space orbital variation (CSOV) method to obtain and analyze the HF and DFT total energies of cluster mode ls representing the interaction of CO and NH3 on Cu(100) and Pt(lll). The q ualitative picture of the chemisorption bond arising from ab initio HF and DFT quantum-chemical approaches is essentially the same; the relative impor tance of the different mechanisms remains unchanged. The main quantitative effect of electronic correlation is to increase the inter-unit charge trans fer but substrate polarization is also affected. A very important consequen ce emerges: it is not necessary to revise the physical mechanisms that have been previously proposed from the ab initio Hartree-Fock cluster model app roach. (C) 1999 Elsevier Science B.V. All rights reserved.