Accurate Evaluation of Real-Time Density Functional Theory Providing Access to Challenging Electron Dynamics.

We demonstrate that electronic excitations and their transition densities can be obtained from real-time density functional theory calculations with great accuracy by relating the data from numerical propagation to the analytical form of the electronic response after a boost excitation. The latter is derived in this article. This approach facilitates quantitatively obtaining oscillator strengths, identifying excitations that carry very small oscillator strengths, and assessing electronic couplings from transition densities based on comparatively short propagation times. These features are of interest in particular for studying light-harvesting systems. For demonstration purposes, we study the Q band excitations of bacteriochlorophyll a (BChl a) and calculate coupling strengths between two BChl a's to check the validity of the dipole-dipole and pure Coulomb coupling mechanisms. For further illustration, we investigate the paradigm test system Na4 and the coupling between two Na2 dimers.