  Time-dependent density-functional theory (TDDFT) often successfully
reproduces excitation energies of finite systems, already in the adiabatic
local-density approximation (ALDA). Here we show for prototypical molecular
materials, i.e., oligothiophenes, that ALDA largely fails and explain why this
is so. By comparing TDDFT with an in-depth analysis based on many-body
perturbation theory, we demonstrate that correlation effects crucially impact
energies and character of the optical excitations not only for molecules of
increasing length and in crystalline environment, but even for isolated small
molecules. We argue that only high-level methodologies, which explicitly
include correlation effects, can reproduce optical spectra of molecular
materials with equal accuracy from gas phase to crystal structures.
