Ab initio calculations performed on Co adsorption on the Fe(100) surface and complementing theoretical techniques

Abstract

Unrestricted Hartree-Fock calculations were performed on FexCO clusters to model the CO(α1), CO(α2), and CO(α3) adsorptions on the Fe(100) surface. Clusters of FeCO(C4v) and a multiplicity of 5, Fe2CO(C2v) and a multiplicity of 7, and Fe2CO(Cs) and a multiplicity of 7, were constructed to model, respectively, the adsorption for the on top site, bridging site, and tilted CO structure at the 4-fold site. The CO position was optimized with respect to the Fe bulk distances using gradient techniques and the partial geometry optimization. CO stretching frequencies were calculated for each optimized geometry, and we find no evidence supporting CO adsorption in the bridging site. Using a full basis set the calculated CO stretching frequencies for the FeCO(C4v), Fe2CO(C2v), and the Fe2CO(Cs) clusters are 1992, 1767, and 771 cm^-1, respectively. The CSOV analysis was executed to analyze the major orbital interactions between the CO and Fex clusters. For both Fe2CO clusters, the CO π* perpendicular to the Fe2 axis had a more significant contribution involving the n backdonation from the Fe2 clusters. Furthermore, the spin minority d electrons are mainly responsible for the n backdonation. Due to problems with SCF convergence' incurred during the FexCO studies, we were forced to investigate a number of different techniques to achieve SCF convergence. Therefore, techniques that generate starting guesses of the eigenvectors for the SCF procedure and techniques used to accelerate SCF convergence are reviewed. The standard guesses of Hcore and charge build-up are examined, and we introduce a new incremental cluster method for generating starting guesses for larger clusters. The standard techniques of extrapolation, DIIS, damping, level shifting, restrict, and symmetry blocking are examined, and we also developed the hacker method and partial geometry optimization as new techniques to achieve SCF convergence. Results of the review indicate that the most important element for obtaining SCF convergence is the starting guess. A procedure for performing transition metal cluster calculations is outlined.