Efficient ab initio calculations of bound and continuum excitons in the absorption spectra of semiconductors and insulators

We present calculations of the absorption spectrum of semiconductors and insulators comparing various approaches: 共i兲 the two-particle Bethe-Salpeter equation of many-body perturbation theory; 共ii兲 time-dependent density-functional theory using a recently developed kernel that was derived from the Bethe-Salpeter equation; and 共iii兲 a mapping scheme that we propose in the present work and that allows one to derive different parameter-free approximations to 共ii兲. We show that all methods reproduce the series of bound excitons in the gap of solid argon, as well as continuum excitons in semiconductors. This is even true for the simplest static approximation, which allows us to reformulate the equations in a way such that the scaling of the calculations with the number of atoms equals the one of the random phase approximation.