Adsorption and Diffusion of Gallium Adatoms on the Si(100)-2 x 1 Reconstructed Surface: A Multiconfiguration Self-Consistent Field Study Utilizing Molecular Surface Clusters

Ab initio electronic structure theory was used to model systems that depict Ga and Ga(2) adsorbed on the Si(100)-2 x 1 reconstructed surface. The prototypical Si(15)H(16) molecular cluster based on quantum mechanics (QM) was used to model the Si(100)-2 x 1 reconstructed surface. A larger Si(199)H(92) molecular cluster based on a hybrid quantum mechanics molecular mechanics (QM/MM) methodology was used to incorporate bulk substrate effects on the adsorbed species. Since the Si(100)-2 x 1 reconstructed surface is comprised of Si dimers that exhibit significant diradical character, multiconfiguration self-consistent field (MCSCF) methodology was used to treat the relevant potential energy surfaces. Hessian calculations were used to characterize all structures, while intrinsic reaction coordinate (minimum energy path) computations were performed to validate the potential energy surface. Dynamic correlation effects were computed at MCSCF optimized structures by multireference second-order perturbation theory. Results from the two cluster models were compared to assess the need to include bulk effects in the surface model.