PERFORMANCE OF CORRELATION FUNCTIONALS IN AB-INITIO CHEMISORPTION CLUSTER-MODEL CALCULATIONS - ALKALI-METALS ON SI(111)

The performance of different correlation functionals has been tested f or alkali metals, Li to Cs, interacting with cluster models simulating different active sites of the Si(111) surface. In all cases, the ab i nitio Hartree-Fock density has been obtained and used as a starting po int. The electronic correlation energy is then introduced as an a post eriori correction to the Hartree-Fock energy using different correlati on functionals. By making use of the ionic nature of the interaction a nd of different dissociation limits we have been able to prove that al l functionals tested introduce the right correlation energy, although to a different extent. Hence, correlation functionals appear as an eff ective and easy way to introduce electronic correlation in the ab init io Hartree-Fock description of the chemisorption bond in complex syste ms where conventional configuration interaction techniques cannot be u sed. However, the calculated energies may differ by some tens of eV. T herefore, these methods can be employed to get a qualitative idea of h ow important correlation effects are, but they have some limitations i f accurate binding energies are to be obtained.