Excitonic DFT: An Efficient and Flexible Constrained DFT Approach for Simulating Neutral Excitations in Isolated and Periodic Systems

State-of-the-art methods for calculating neutral excitation energies, such as time-dependent density functional theory (TDDFT) and GW + Bethe-Salpeter, are computationally demanding and currently limited to moderate system sizes. Recent years have seen a growing interest in adapting density functional theory (DFT) to describe excited states, in the form of methods including ensemble DFT [1], constricted variational DFT [2], and restricted open-shell Kohn-Sham [3]. We introduce a computationally light, generally applicable, first-principles approximation based on constrained DFT [4-5] for calculating neutral excitations, and our implementation in the linear-scaling DFT code ONETEP. We demonstrate close agreement with TDDFT for the Thiel molecular test set [6], and show that our method may also be successfully applied to extended systems like semiconductors and 2D materials.

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[5] D. Turban et al, Phys. Rev. B 93, 165102 (2016)
[6] D. Jacquemin et al, J. Chem. Theory Comput. 5, 2420 (2009)