Calculating Off-Site Excitations in Symmetric Donor-Acceptor Systems via Time-Dependent Density Functional Theory with Range-Separated Density Functionals

Time-dependent density functional theory with range-separated hybrid functionals is used to calculate off-site excitations involving transitions between spatially separated orbitals in weakly coupled systems. Although such off site excitations involve charge transfer, orbital degeneracy in symmetrical systems results in linear combinations of off site excitations with equal weights and therefore zero net charge-transfer character. Like other types of off-site excitations, such "hidden" charge-transfer excitations are not accurately captured by conventional density functionals. We show that the recently introduced Baer-Neuhauser-Livshitz range-separated hybrid functional accurately characterizes such hidden off-site excitation energies via applications to the ethene dimer model system and to dye-functionalized silsesquioxanes.