An efficient method for calculating molecular excitation energies by time-dependent density-functional theory

We present a new efficient method for determining excitation energies and oscillator strengths of molecules which is based on time-dependent density functional theory (TDDFT). It yields the valence excitation spectrum of a molecule at a fraction of the computational effort necessary for the corresponding ground state calculation. The efficiency of the method originates in the extensive use of auxiliary basis sets. Nevertheless this approach does not lead to significant inaccuracies. The method is applied to benzene, naphtalene, and the higher linearly annelated polycyclic aromatic hydrocarbons anthracene to heptacene. Features and trends in the excitation spectra of these aromatic hydrocarbons are well described by the new TDDFT method. The standard basis sets commonly used in TDDFT based procedures are found to be inadequate for an accurate treatment of excitations into Rydberg-type states. (C) 2000 American Institute of Physics. [S0021- 9606(00)30530-X].