\textit{Ab initio} Optical Absorption by A Simple and Efficient Method: Single Excitation Configuration Interaction After Downfolding

We present a simple and efficient \textit{ab initio} method for calculating electronic excited states and optical absorption spectra of solids. The method is based on a single-excitation configuration-interaction calculation after downfolding to model Hamiltonians represented by maximally-localized Wannier functions. Single-excitation configurations are crucially important in evaluating a linear absorption, because they can describe a so-called excitonic effect; interactions in electron-hole pairs generated by excitations. A test was performed for a semiconductor GaAs, and detailed analyses for the resulting spectra and single-excitation many-body wavefunctions are presented. This work is supported by a Grant-in-Aid for Scientific Research in Priority Areas, ``Development of New Quantum Simulators and Quantum Design'' (No. 17064004) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.