Excitons in solids, at a (TD)-DFT cost

The ab initio GW - Bethe-Salpeter equation (BSE) approach has become standard for predicting and understanding excitons in condensed matter. Here we perform a detailed analysis of exciton states using linear response time-dependent density functional theory (TD DFT). We investigate a complex bulk halide double perovskite (Cs₂AgBiBr₆), two prototypical reconstructed surfaces (Si (111)-(2x1) and Ge (111)-(2x1)), and a van der Waals material (MoS₂) with various exchange correlation functionals, comparing where possible with existing ab initio GW-BSE calculations. We show that an analysis on par with GW-BSE is possible with TD DFT, at a highly reduced cost, for specific functionals.