A Full Minimal Coupling GW-BSE Framework for Circular Dichroism in Solids: Applications to Chiral 2D Perovskites

Abstract: Circular dichroism (CD) and other chiroptical responses are a key probe of both chirality and momentum-space geometry in solids, but first-principles calculations are still challenging in periodic systems with strong exciton effects. Here, we develop a gauge-invariant first-principles framework for CD including exciton effects based on full minimal coupling (FMC) within the GW plus Bethe-Salpeter equation (GW-BSE) formalism. In contrast to standard multipole expansion and sum-over-states (SOS) approaches, which require careful gauge-fixing, converge slowly, and suffer origin ambiguities, FMC evaluates optical matrix elements directly at finite photon wavevector, naturally including intraband and near-degenerate transitions while placing electric-dipole (ED), magnetic-dipole (MD), and electric-quadrupole (EQ) contributions on equal footing. Applied to two prototypical two-dimensional chiral hybrid perovskites, (S-NEA)2PbBr4 and (S-MBA)2PbI4, our calculations reveal that MD and EQ channels contribute equally to the CD signal. Crucially, intraband and quasi-degenerate transitions only captured within FMC can significantly modify CD spectra, especially in systems with dense band degeneracies. The FMC framework, therefore, offers a computationally efficient and numerically robust way for predicting chiral optoelectronic phenomena in complex solids.