Assessing the Bethe-Salpeter Equation within the GW Approximation (GW/BSE) for Excitation Energies of BODIPY and Aza-BODIPY Derivatives

Boron-dipyrromethenes (BODIPYs) are a class of fluorescent molecules known for their highly tunable optical properties and diverse array of applications, ranging from medical imaging to photovoltaics. Due to multireference behaviour, strong electron correlation, and double excitation character, excited states of these systems are characteristically difficult to model computationally, complicating the theoretical design of novel BODIPY derivatives. Time-dependent density functional theory (TD-DFT) is known to consistently overestimate BODIPY excitation energies, producing mean absolute error (MAE) values often exceeding 0.3 eV. To assess an alternative approach, we turn to the Bethe-Salpeter equation within the GW approximation (GW/BSE), which has recently been gaining traction for its ability to eliminate many common issues of TD-DFT (such as the self-interaction error, unequal treatment of local and charge-transfer excitations, and strong functional dependence) at an equivalent computational cost. Using experimental excitation energies for a set of 17 substituted BODIPYs and aza-BODIPYs, we present a comprehensive benchmark of the GW/BSE method. The assessment includes analysis of basis set convergence with the (aug)-cc-pVnZ (n = D, T, Q) basis set series, comparison of evGW and G0W0 schemes to evaluate the effect of self-consistency in the GW calculation, and evaluation of the impact of different DFT starting points (PBE, LC-ωPBE, and PBE0 with varying levels of exact exchange). Results demonstrate that GW/BSE with an appropriately selected Kohn-Sham starting point can produce MAE values under 0.1 eV while maintaining strong linear correlations, significantly outperforming TD-DFT and cementing GW/BSE as a valuable tool for the prediction of BODIPY excitation energies and design of future BODIPY derivatives.