Large Scale Bethe-Salpeter Equation Calculations*

We present an efficient method to solve the Bethe-Salpeter equation (BSE) to compute electron-hole excitations and optical spectra of systems with hundreds of electrons, including molecules and solids. The method does not require the explicit evaluation of virtual electronic states, nor the inversion of dielectric matrices; its computational workload scales as the third power of the number of electrons in the system. We computed density response functions in finite field, and used localized Kohn-Sham orbitals, as obtained by the bisection method [1]. The approach is implemented by coupling the Qbox [http://qboxcode.org] and WEST [http://www.west-code.org] codes. Results for the excitonic binding energies and optical absorption spectra of the Thiel molecular set, and for condensed systems of unprecedented size, including water and ice samples with hundreds of electrons, will be discussed. Ref. [1] François Gygi, PRL 102, 166406 (2009).