Proton Quantum Effect on Electronic Excitation Facilitated by Hydrogen Bonds: BSE@GW with Nuclear Electronic Orbital Method

Eumelanin, an organic crystal, is composed of intricate structures formed by stacking indole derivative oligomers, including 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Recent work has unveiled that the remarkable optical absorption of DHI is predominantly influenced by π-stacking interactions, potentially associated with non-covalent hydrogen-bonding interactions. Notably, the structural components of DHI provide insights into the directional impact of hydrogen bonding on elucidating exciton localization and delocalization within the stacked units in DHI crystals To gain detailed understanding of this phenomenon at the molecular level, we investigate the quantum effects of protons on this intriguing excitonic behavior driven by hydrogen bonding using our recently developed all-electron Bethe-Salpeter equation (BSE) @ GW implementation in FHI-aims code, combined with the nuclear electronic orbital method.