A finite-field approach to performing GW calculations and solving the Bethe-Salpeter equation

We describe a finite field approach to compute density response functions, and to compute optical spectra and exciton binding energies of molecules and solids based on the solution of Bethe-Salpeter equation. The approach
allows for efficient G0W0 and G0W0Γ calculations as well as for the calculation of optical spectra beyond the random phase approximation.
In addition it utilizes localized orbitals obtained from Bloch states with bisection techniques, thus greatly reducing the complexity of the calculation and enabling the efficient use of hybrid functionals to compute input orbitals, when using a plane-wave representation.