ALL ELECTRON VERSUS PSEUDOPOTENTIALS IN ABINITIO CHEMISORPTION CLUSTER MODEL-CALCULATIONS

The interaction of atomic oxygen with a Cu5 cluster model stimulating chemisorption on the fourfold site is studied at the ab initio SCF level. Three different levels of approximation were tested. In the first one all the electrons were explicitly included while in the other two only the 3d(10)4s1 or the 4s1 electrons were explicitly included, the corresponding Cu cores were treated by means of a nonempirical pseudopotential. We have obtained Cu5O wave functions where only the Cu 4sp or the 4sp plus the 3d electrons are varied and deeper core electrons are frozen as well as wave functions where all the Cu electrons are varied. This allows us to separately examine the effect of the pseudopotentials on the Cu 4sp and Cu 3d contributions to the Cu-O bond. It is found that the pseudopotential errors for these individual contributions are about 0.25-0.50 eV. These uncertainties are sufficiently large so that they limit the use of pseudopotentials to providing qualitative rather than quantiative information about the bond. Finally, a proposal is made for the proper way to obtain the Cu-O binding energy with SCF wave functions for clusters.