Time dependent DFT in natural orbitals

Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. The discretization of the electronic wave functions on a spatial grid and the use of absorbing boundary conditions allows one to study electronic emission, and in particular photoelectron spectra. However, in this scheme, the unitarity of the time propagation is lost, and the set of propagated wave functions is not orthonormal anymore. This constitutes a major obstacle for extensions beyond TDDFT, e.g., to include dissipation from electronic correlations. In this work, we demonstrate that the time propagation can be done alternatively in an orthonormal basis, the so-called set of “natural orbitals”, with results very similar to the usual time propagation, even for the more involved photo-emission spectrum. This technique will serve as a basis for including dynamical correlations.