Comparison of the electronic excitation spectra of chlorophyll a and pheophytin a calculated at density functional theory level

The molecular structures of chlorophyll a and pheophytin a, with the phytyl group replaced by a hydrogen, have been optimized at the density functional theory (DFT) level. The excitation energies and oscillator strengths obtained at the DFT level using the time-dependent perturbation theory approach are compared to experimental data. In the visible region, 12 electronic transitions are obtained which is in a good agreement with the experimental observations. The present calculations show that Goutermanʹs generally accepted four-orbital model does not correctly describe the excitation spectrum of chlorophylls.