Electronic band structure of fluorite

The electronic band structure of Ca${\mathrm{F}}_2$ is calculated by a mixed tight-binding orthogonalized-plane-wave method. The tight-binding method includes the calculation of three-center terms and the plane waves are then orthogonalized to the occupied levels. The exchange potential is of the Slater form together with a scaling parameter $\lambda$ adjusted to the gap. The obtained band structure is found to differ significantly with that of previously proposed empirical schemes. Comparison is made with photoemission and optical data and the agreement is good.