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We examine whether essence and quantitative aspects of electronic excitationspectra are correctly captured by an effective low-energy model constructedfrom an {\em ab initio} downfolding scheme. A global electronic structure isfirst calculated by {\em ab initio} density-functional calculations with thegeneralized gradient approximation. With the help of constrained densityfunctional theory, the low-energy effective Hamiltonian for bands near theFermi level is constructed by the downfolding procedure in the basis ofmaximally localized Wannier functions. The excited states of this low-energyeffective Hamiltonian ascribed to an extended Hubbard model are calculated byusing a low-energy solver. As the solver, we employ the Hartree-Fockapproximation supplemented by the single-excitation configuration-interactionmethod considering electron-hole interactions. The present three-stage methodis applied to GaAs, where eight bands are retained in the effective model afterthe downfolding. The resulting spectra well reproduce the experimental results,indicating that our downfolding scheme offers a satisfactory framework of theelectronic structure calculation, particularly for the excitations and dynamicsas well as for the ground state.

Optical Absorption Study by Ab initio Downfolding Approach: Application to GaAs