State-of-the-art computational approaches to describe photophysical and photochemical processes in extended materials most often rely on linear response time-dependent density functional theory, representing a well-established tool with high feasibility and robustness to enable broad applications. However, when aiming for high-throughput screening on the large scale, the computation of excited-state properties becomes tedious, being in comparison to ground-state properties theoretically more challenging, requiring an accurate description of exact exchange, as well as computationally more demanding with a less favorable scaling and thus higher computational cost. Aiming for a pragmatic and efficient pre-screening tool for the large-scale characterization of porous framework materials, we propose a combined hybrid functional and semi-empirical tight binding scheme exploiting the concepts of the auxiliary density matrix method (ADMM) and the simplified Tamm-Dancoff approximation (sTDA). Among the existing techniques to improve the unfavorable basis-set dependence and thus high computational cost of exact Hartree-Fock exchange, ADMM has proven to be a reliable approach to achieve significant speedups while retaining sufficient accuracy, being based on the idea of constructing an auxiliary density matrix for the evaluation of the exact exchange term and correcting the thereby introduced error by a density functional contribution. The setup for the reference ground-state treatment is complemented by employing the sTDA tight binding ansatz for the excited state: sTDA relies only on global, atom-specific parameters requiring no pair-wise or bond-specific parameterizations while being at the same time computationally cheap and physically reasonable. It employs semi-empirical electron repulsion operators and in general thus goes along with a minor loss in accuracy, but keeps or even improves the description of long-range electrostatic interactions as required for charge-transfer or Rydberg excitations. The implementation of the combined hybrid-ADMM and sTDA approach in CP2K is benchmarked for molecular test sets and a series of covalent organic frameworks and results regarding the description of absorption as well as emission spectroscopy are presented assessing the performance for excited-state gradients, adiabatic excitation energies and corresponding spectra.

Increasing efficiency for theoretical absorption and emission spectroscopy of extended systems on a large scale

ACS Spring 2022

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