Excited-state force calculations from GW/BSE and DFPT: development and application to organic metal halide perovskites

While absorption of light has been long studied in electronic structure, the interactions of the resulting excited states with the lattice that cause light-induced structural changes remain hard to handle. This is an underlying phenomenon for photodegradation, Stokes shifts, exciton transport, and other photophysics, which can be studied via excited-state forces. Ismail-Beigi and Louie [Phys. Rev. Lett. 90, 076401 (2003)] developed an approximate theory combining quasiparticle and excitonic effects from GW and Bethe-Salpeter Equation (BSE) with electron-phonon interactions from Density Functional Perturbation Theory, but this approach has been little used. We revisit this theory, with improvements to the underlying approximations, and implement it in a practical workflow for BerkeleyGW. We make detailed tests of the validity of these approximations and demonstrate favorable convergence properties that make the forces less time-consuming than ordinary GW/BSE. Then we explore some applications to methylammonium lead iodide perovskites, whose use in solar cells is limited by photodegradation. We study the equilibrium structures of the excited states, and coupling between different excitons due to the phonons, to give insight into the photophysics of perovskites.