Lars Hedin and the quest for a theory of excited states

Abstract

The theory of excited electronic states is central to a quantitative understanding of the interaction between light and matter and hence to calculations of various x-ray and optical spectra. Although ground state properties are well described by density functional theory (DFT), that theory is inappropriate for excited states, for which a fully quantitative theory has yet to be developed. No one has contributed more to than Lars Hedin to the quest for such a theory. Here we briefly review a number of his key contributions. One of the most influential is the “GW” approximation for the self-energy of an excited electron. Another is the “blue-electron” approximation for photon and electron spectroscopies. A third, which is relevant to the topics of this conference, is a many-body theory of x-ray spectra based on the “quasi-boson” approximation. This approach is a generalization of the GW approximation which takes into account both extrinsic and intrinsic losses as well as the interference between them, and simplifies the treatment of losses and the core-hole interaction. What makes Hedin's theories stand out is their mathematical elegance and physical robustness, based on ideas conceived before their full implementation was practical. With today's advances in computational power, however, the impact of Lars' work is becoming ever more widely appreciated.