Conceptual DFT and Excited States
Abstract
5.1 Introduction Conceptual density functional theory (DFT) [1-3], by its explicit dependence over the first Hohenberg-Kohn theorem [4], is a ground state theory. Therefore, the very purpose of this chapter may then seem out of scope of conceptual DFT. How can one include excited states in a ground state only representation? And for which aim? Several approaches were undertaken in the past years to embrace this topic, which we propose to revisit here. Broadly speaking, two axes can be drawn: either one is interested in characterizing the excited states properties, by studying them or conjecturing their properties from the ground state, or either one is interested in using the excited states to improve the representation of the ground state. Illustrations of both ideas are provided in Sections 5.1 and 5.2, respectively. Note, however, this chapter does not aim at being exhaustive. Noticeably, many developments were proposed over the years, especially at times when no efficient computational framework for excited states was available (before the advent of time-dependent density functional theory [TDDFT], so to say). Additionally, some works focused on the development of time-independent DFT models, looking for the definition and construction of either state-specific or generic energy density functionals [5, 6]. We believe such developments, though precious and meaningful, fall out of the scope of the chapter, and for the sake of clarity and concision we decided not to include them in the following. 5.2 Reactivity and Selectivity of Excited States 5.2.1 Photochemical Reactivity Study of excited states properties is particularly interesting for photochemical reactions (reactions performed under photoexcitation) [7]. Experimentally, it is indeed
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