Optical absorption of diamond nanocrystals from ab initio density-functional calculations

Absorption spectrum of small nanodiamonds, i.e., diamondoids has been recently measured exhibiting features that are not understood. Previous calculations, even beyond standard density-functional theory (DFT), failed to obtain the experimental optical gaps (E_g) of diamondoids. We show that all-electron time-dependent DFT (TD-DFT) calculations including hybrid functional in the TD-DFT kernel are able to provide quantitatively accurate results. Our calculations demonstrate that Rydberg transitions are characteristic even for relatively large nanodiamonds resulting in low E_g . The nonmonotonic size dependence of E_g is explained by symmetry considerations.