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Successes and failures of time-dependent density functional theory for the low-lying excited states of chlorophylls

It has been suggested that electronic excited states outside of those included within the Gouterman four-orbital model (Q y , Q x , B x , and B y ) are present in the low-energy region of chlorophyll molecules. In particular, studies performed using time-dependent density functional theory (TDDFT) predict the presence of a very large number of such states, most of whom are expected to be ‘dark’ states that are thus difficult to identify. So far, there has been no experimental test that has authoritatively confirmed or contradicted these predictions. The paper considers the response of the TDDFT-calculated excited states to applied electric fields of the order of those experienced by the chlorophylls in Photosystem-I (PS-I). TDDFT calculations using the PW91 density functional with the 6-31G* basis set are shown to predict that charge-transfer bands form the lowest excited state under these conditions, contrary to experimental results. In general, TDDFT calculations using modern functionals are shown to be unreliable for calculations of the excited states of chlorophylls, although important, realistic spectral information is depicted within them that may possibly be extracted for useful purposes.