Electronic structure and optical properties of solid methane

Abstract

The electronic structure of solid methane, including the virtual levels, is studied in a self-consistent Hartree-Fock model as a function of the lattice constant. It is found that the band gap is a sensitive function of pressure. It is also found that the resulting accuracy of the conduction bands depends heavily upon the quality of the virtual orbitals, a fact not appreciated in previous studies on solid C${\mathrm{H}}_4$. It is found essential to include correlations explicitly in order to facilitate a comparison to experiment. This is done by means of perturbation theory. The inclusion of Coulomb correlations causing the formation of excitons in the excited spectra is accomplished by means of degenerate perturbation theory in the continuum. Results are compared to experiment.