Ab Initio Electronic Structure Calculations
The adsorption of different molecules on surfaces is studied using ab initio embedded cluster calculations. Our goal is the analysis of the bonding mechanisms and the calculation of reliable potential energy surfaces for the electronic ground state of the cluser-adsorbate-complex. Systems under the present study are NO on NiO(100), CO on Cr2O3(0001) and CO/Pd(111).
One main focus of our theoretical studies is the calculation and characterization of excited states of molecules on surfaces. For the first time it has been possible to calculate the electronic states which are involved in the laser induced desorption of NO from NiO(100) and CO from Cr2O3(0001) by employing large scale CI-calculations for an embedded cluster model. Four-dimensional potential energy surfaces have been constructed and allow for a fundamental insight into the mechanism and driving forces of DIET (Desorption Induced by Electronic Transition)-processes on oxide surfaces.
The lifetime of wave packets in excited states, involved in the laser induced desorption of molecules from surfaces, is determined by non-adiabatic quenching of the intermediate species. Because of our knowledge of the wave function for the intermediate excited states, it should be possible to obtain an ab initio description of the relaxation process by calculating diabatic potential surfaces and coupling elements. The method of L.S. Cederbaum et al. has been implemented in our ab initio-CI-code and was succesfully applied to the benchmark system HeH+.
In collaboration with the group of E.A. Carter (UCLA) we take part in the development of a new embedding theory which efficiently bridges the gap between periodic density functional claculations (DFT) and conventional ab initio quantum chemistry. A local region of the adsorbate-surface complex is treated by accurate quantum chemical methods (CI, CASSCF, MP-n), whereas the infinite background of the solid is calculated by DFT. From the DFT calculation we obtain an effective one-electron embedding operator. This method can be regarded as a local correction to DFT and allows the accurate treatment of electron correlation effects. Recently, we were able to extend the embedding scheme to localized excited states on surfaces including periodic boundary conditions, a heretofore intractable goal.