Ab initio calculation of ε2(ω) including the electron-hole interaction:: Application to GaN and CaF2

We present a computationally efficient first-principles scheme to calculate epsilon(2)(omega) for crystalline insulators, including the electron-hole interaction. The effective Hamiltonian for electron-hole pairs contains both the exchange and direct parts of this interaction. An iterative scheme is used in which the omega moments of epsilon(2)(omega) are computed by repeated action of the Hamiltonian on electron-hole pair states. The scheme is applied to two insulators where there are significant experimental uncertainties in their ultraviolet optical properties: GaN and CaF2. [SO163-1829(99)08507-0].