Efficient Excitonic Configuration Interaction for Large-Scale Multichromophoric Systems Using the Resolution-of-Identity Approximation

The calculation of electronic excited states in extended multichromophoric systems is computationally challenging. Here, we accelerate our recently introduced excitonic configuration interaction (ECI) method [T. Piteša et al. J. Chem. Theory Comput. 2024, 20, 5609] with the resolution-of-identity approximation for the two-site two-electron integrals in the calculation of the interchromophoric Coulomb and exchange terms. Additionally, a simple overlap-based scheme is introduced to prescreen the Cholesky-transformed tensor of the three-centric two-electron interchromophoric exchange integrals, significantly accelerating the expensive tensor contraction for the two-site exchange term. This reduces both cost and memory requirements, enabling large-scale calculations of systems with many chromophores. We demonstrate its efficiency and accuracy by calculating electronic excited states of chains of up to 32 BODIPY chromophores and networks of up to 100 peri-xanthenoxanthene units, with 12 320 and 43 600 basis functions, respectively. We achieve errors in the excitation energies below 30 meV, using site states calculated with time-dependent density functional theory.