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.