Intramolecular electron localization and local-density calculations on silicon-containing molecules: Tetramethylsilane and hexamethyldisilane

We report discrete-variational method Hartree-Fock-Slater (HFS) calculations on the title compounds. For a self-consistent potential based upon a Mulliken superposition of spherical charge densities (the self-consistent-charge scheme) the orbital energetics exhibit substantial disagreement with experiment; inclusion of more radial structure (and, less effectively, of p waves) in the self-consistent potential produces excellent experimental agreement, and permits us to assign the lowest optical excited state as an e_u π-type state with some Si d-function character. This is essentially in agreement with an early experimental assignment of Gilman. Electron delocalization in silane oligomers is rationalized on the basis of empty orbital energetics and spatial structure. For proper description of diffuse molecular states and optical excitations, we emphasize the necessity of utilizing a sufficiently flexible representation of the molecular potential.