Optical saturation driven by exciton confinement in molecular-chains: a TDDFT study

TL;DR

This study uses advanced TDDFT calculations to show that excitonic confinement in one-dimensional molecular chains causes the saturation of their polarizability, highlighting the importance of nonlocal effects in accurate modeling.

Contribution

It introduces a new exchange-correlation kernel for TDDFT that captures excitonic confinement effects in molecular chains, improving understanding of their optical properties.

Findings

Excitonic confinement drives polarizability saturation.

Simple local functionals lack memory and ultranonlocality effects.

Performance of simple functionals improves as the system gap decreases.

Abstract

We have identified excitonic confinement in one-dimensional molecular chains (i.e. polyacetylene and H$_2$) as the main driving force for the saturation of the chain polarizability as a function of the number of molecular units. This conclusion is based on first principles time--dependent density functional theory calculations performed with a new derived exchange--correlation kernel. The failure of simple local and semi--local functionals is shown to be related to the lack of memory effects, spatial ultranonlocality, and self--interaction corrections. These effects get smaller as the gap of the system reduces, in which case such simple approximations do perform better.