An efficient exciton coupling scheme based on simplified time-dependent density functional theory

A very efficient and broadly applicable exciton coupling (ExC) approach based on simplified time-dependent density functional theory (sTD-DFT) is presented. Starting from this parent method, non-overlapping fragments and neglect of interfragment charge transfer excitations are assumed to arrive at the ExC procedure. This leads to an ExC Hamiltonian that pro- vides equivalent electronic absorption and circular dichroism spectra as the parent sTD-DFT method for largely separated fragments. The ExC approach easily accelerates the computa- tion of such spectra of molecular aggregates by about two orders of magnitude compared to sTD-DFT. The latter itself is already faster by about 4–5 orders of magnitude compared to regular TD-DFT. We demonstrate the performance of the approach for excitation spectra of organic molecular clusters. Given that the fragment electronic structure in the ExC-sTD- DFT approach is solved independently, computation of spectra for systems with ∼10,000 atoms can be performed within minutes of computation time. Furthermore, the role of electrostatic embedding in the independent fragments is investigated. For the purposes cov- ered in this work, the embedding can be simplified by employing a dielectric continuum, thus, greatly reducing the overall computational complexity. This approach may be used in screening photophysical properties of large molecular aggregates and soft matter materi- als. We present the derivation and implementation for the Tamm-Dancoff-approximated and the random-phase-approximation eigenvalue problems. Benchmarks compared to the parent sTD-DFT methods are shown for absorption and electronic circular dichroism spectra.