Electronic structure of excited states (a mathematical treasure trove from ensemble density functional theory)

Tim Gould, Griffith University, Queensland, Australia

Monday, December 12,  2022, 14:30 AM, Los Angeles Seminar Room

Abstract: Density functional theory (DFT) can yield excited state properties as functionals of the density, by working from ensemble instead of pure states. Facilitating excited states requires exact functionals to be carefully generalized to weight-dependent forms, to avoid spurious interactions from misapplied ansatze. This talk will discuss recent advances on rigorous definitions for the kinetic, Hartree and exchange [1] and correlation [2] ensemble energy functionals, which together yield a complete generalisation of pure state DFT. These works reveal unusual (yet tractable) properties of density functionals -- notably the need for multi-configurational Kohn-Sham states, and strong absence of weight-dependence in the low density limit. Ensemble generalized Kohn-Sham theory based on these functionals, and its successful applications to excited state chemical problems [3], will finally be discussed.

1. Hartree and Exchange in Ensemble Density Functional Theory: Avoiding the Nonuniqueness Disaster,Tim Gould and Stefano Pittalis, Phys. Rev. Lett. 119, 243001 – Published 13 December 2017
2. Electronic excited states in extreme limits via ensemble density functionals:Tim Gould, Derk P. Kooi, Paola Gori-Giorgi, Stefano Pittalis, https://doi.org/10.48550/arXiv.2205.07136. 
3. Single Excitation Energies Obtained from the Ensemble “HOMO− LUMO Gap”: Exact Results and Approximations. Tim Gould,* Zahed Hashimi, Leeor Kronik, and Stephen G. Dale. https://doi.org/10.1021/acs.jpclett.2c00042