First-principles GW–BSE excitations in organic molecules

We present a first-principles method for the calculation of optical excitations in nanosystems. The method is based on solving the Bethe–Salpeter equation (BSE) for neutral excitations. The electron self-energy is evaluated within the GW approximation, with dynamical screening effects described within time-dependent density-functional theory in the adiabatic, local approximation. This method is applied to two systems: the benzene molecule, C6H6, and azobenzene, C12H10N2. We give a description of the photoisomerization process of azobenzene after an n–π* excitation, which is consistent with multi-configuration calculations.