Dynamical and nondynamical correlation effects in a b i n i t i o chemisorption cluster model calculations. Ground and low lying states of H on Cu(100) and Ag(100)

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Abstract
The nondynamical correlation effects for the interaction between an adsorbate and a surface cluster model can be recovered by means of a simple and accurate method. A projection localization procedure permits to distinguish between cluster and adsorbate MOs and between lone pairs or bonds. Valence virtual MOs are obtained by an inexpensive method. Once the valence space is defined CASCI calculations account for these correlation effects, the results being in good agreement with CASSCF calculations. The dynamical correlation effects are taken into account by using a perturbative approach in which the CAS wave function is taken as the zeroth order one. Results, thus obtained, are also in agreement with accurate CIPSI calculations. The overall procedure is illustrated by means of calculations on the ground and low lying electronic states of the on top chemisorption of atomic hydrogen on the Cu5 and Ag5 cluster models simulating the (100) metal surface. The main effects of nondynamical correlation in the electronic ground state are a slight increase on the internuclear equilibrium distance of the adsorbate to the surface model and a considerable contribution to the binding energy (≊20% of the total binding energy). The dynamical effects tend to cancel the effect on the equilibrium distance and left almost unchanged the binding energy. Since these effects are quite small valence CASCI can lead to an accurate enough description of the chemisorption bonds. These general trends also hold for the excited states studied here.