Frequency-dependent response properties and excitation energies from one-electron density matrix functionals

The recent formulation of the time-dependent density matrix functional theory (TD-DMFT) has opened an avenue to calculations of frequency-dependent response properties and excitation energies of atoms and molecules. In practice, the accuracy of the computed data is limited by both the errors inherent to the adiabatic approximation or its modifications and the quality of the energy functionals. The relative importance of these two factors is carefully assessed with test calculations on diatomic molecules with few electrons. The test results clearly demonstrate the superiority of an ad hoc approach that corrects the improper behavior of the adiabatic approximation at the low-frequency limit. Even more importantly, TD-DMFT convincingly removes the ambiguity in the choice of the two-electron integrals that is present in the stationary-state case. On the other hand, paralleling the previously reached conclusions pertinent to ionization potentials, the presently available BBC-type functionals are found to be insufficiently accurate to provide reliable quantitative predictions of excitation energies.