Calculations of excited electronic states using density functionals - without introducing time-dependence

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• Professor Hannes Jónsson, University of Iceland
• Wednesday 30 April 2025, 14:30-15:30
• Unilever Lecture Theatre, Yusuf Hamied Department of Chemistry.

If you have a question about this talk, please contact Lisa Masters.

Calculations of excited electronic states are important in studies of various applications such as light harvesting, photocatalysis and molecular motors. They are challenging as commonly used algorithms are designed to converge on the ground state. As a result, a time-dependent formulation of density functional theory (DFT) is frequently used, TD-DFT, especially within the linear response approximation. This approximate approach, however, has several limitations especially when significant charge transfer occurs during the excitation and when states are close in energy. Within configuration interaction (CI) theory, it is evident that excited states correspond to saddle points on the electronic energy surface, with the saddle point order increasing with the excitation level. While CI calculations can be accelerated greatly by using neural networks [1], they are much too computationally demanding for most problems of interest. DFT is used in most electronic structure calculations carried out today. By using an algorithm for converging on saddle points on the electronic energy surface of a density functional, the orbitals can be optimised for the excited state and provide higher energy solutions to the underlying Kohn-Sham equations [2,3]. This gives more robust estimates of the excited states than TD-DFT while the computational effort is similar to that of a ground state calculation. >>>>>> Several applications of this approach with commonly used density functionals will be presented, as well as calculations using a self-interaction corrected functional that gives improved results. In particular, the various excited states of the ethylene molecule, including the twisting of the C=C double bond, the active element of several molecular motors, and high energy Rydberg states, have been analysed [4]. In a solid state application, the various states relevant for the optical preparation of a pure spin state in nitrogen/vacancy defect in diamond, a system used in various types of quantum technologies such as quantum computing, have been calculated. The results of these calculations are found to be in close agreement with computationally demanding, high-level calculations as well as experiments [5].

[1] Y. L. A. Schmerwitz, L. Thirion, G. Levi, E.Ö. Jónsson, P. Bilous, H. Jónsson and P. Hansmann, (submitted). [2] G. Levi, A.V. Ivanov and H. Jónsson, J. Chem. Theory Comput. 16, 6968 (2020) [3] Y.L.A. Schmerwitz, G. Levi and H. Jónsson, J. Chem. Theory and Comput. 19, 3634 (2023) [4] A.E. Sigurdarson, Y.L.A. Schmerwitz, D.K.V. Tveiten, G. Levi and H. Jónsson, J. Chem. Phys. 159, 214109 (2023) [5] A.V. Ivanov, Y.L.A. Schmerwitz, G. Levi and H. Jónsson, SciPost Physics 15, 009 (2023)

This talk is part of the Theory - Chemistry Research Interest Group series.

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