Density functional theory calculations of the visible spectrum of chlorophyll a

The molecular structure of chlorophyll a including the phytyl chain has been fully optimized at density-functional level. The excitation energies and oscillator strengths obtained at density-functional time-dependent perturbation theory level are in good agreement with experiment. The present calculations show that Goutermanʹs generally accepted four-orbital model does not correctly describe the excitation spectrum of chlorophyll a. Thirteen excited states are found in the visible region: four are found in the long-wavelength region, two weak transitions are obtained at about 490 nm, while seven transitions are in the Soret band region.