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

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.