Perspectives on double-excitations in TDDFT

TL;DR

This paper discusses the limitations of adiabatic TDDFT for double excitations, explores their role in challenging problems, and presents new computational approaches to better capture double-excitation phenomena.

Contribution

It introduces novel methods for calculating double excitations in TDDFT, including autoionizing resonances, quadratic response analysis, and real-time semiclassical approaches.

Findings

Successful calculation of autoionizing resonances in helium

Insights into double excitations in quadratic response

Retrieval of double-excitations via semiclassical real-time methods

Abstract

The adiabatic approximation in time-dependent density functional theory (TDDFT) yields reliable excitation spectra with great efficiency in many cases, but fundamentally fails for states of double excitation character. We discuss how double-excitations are at the root of some of the most challenging problems for TDDFT today. We then present new results for (i) the calculation of autoionizing resonances in the helium atom, (ii) understanding the nature of the double excitations appearing in the quadratic response function, and (iii) retrieving double-excitations through a real-time semiclassical approach to correlation in a model quantum dot.