Assessment of the Ab Initio Bethe-Salpeter Equation Approach for the Low-Lying Excitation Energies of Bacteriochlorophylls and Chlorophylls

Abstract

Bacteriochlorophyll and chlorophyll molecules are crucial building blocks of the photosynthetic apparatus in bacteria, algae and plants. Embedded in transmembrane protein complexes, they are responsible for the primary processes of photosynthesis: excitation energy and charge transfer. Here, we use ab initio many body perturbation theory within the GW approximation and Bethe-Salpeter equation (BSE) approach to calculate the electronic structure and optical excitations of bacteriochlorophylls a, b, c, d and e and chlorophylls a and b. We systematically study the effects of structure, basis set size, partial self-consistency in the GW approach and the underlying exchange-correlation approximation. Furthermore, we compare our calculations with results from time-dependent density functional theory, and calculations and experimental results from the literature, and find that optical excitations calculated with GW +BSE are in very good agreement with experimental data, on average underestimating the Q y and Q x excitations of the entire family of (bacterio)chlorophylls by less than 0.1 eV. Moreover, the GW +BSE approach predicts the energy difference between the two excitations correctly, highlighting the potential of this method for the simulation of larger pigment complexes.