Optical matrix elements have been derived for crystals by applying the theory of optical transitions within the formalism of the linear muffin-tin-orbital band-structure method and using Racah algebra. A Brillouin-zone integration by the tetrahedron method allows straightforward numerical computation of the optical function ε2(ω) [or σ(ω)]. Other optical functions may be derived by using a Kramers-Kronig analysis.
Zemach, R.; Ashkenazi, J.; and Ehrenfreund, E., "Application of the Linear Muffin-Tin-Orbital Band-Structure Method to Calculate Optical Properties of Solids" (1989). Physics Articles and Papers. 37.