We implement the linear response theory within the time-dependent local density-functional formalism (TDLDA) to calculate excitation energies and photoabsorption spectra of atoms and clusters. The calculated ab initio spectra are in very good agreement with experiment. Along with the exact TDLDA formalism, we consider several approximate expressions for the electronic transition energies. The analysis of the spectra calculated with different levels of approximation indicates the important role of the collective electronic excitations in atoms and clusters.

• Received 26 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1919