The formation energies and electronic structure of europium doped zinc oxide has been determined using DFT and many-body $GW$ methods. In the absence of intrisic defects we find that the europium-$f$ states are located in the ZnO band gap with europi
um possessing a formal charge of 2+. On the other hand, the presence of intrinsic defects in ZnO allows intraband $f-f$ transitions otherwise forbidden in atomic europium. This result coorroborates with recently observed photoluminescence in the visible red region [1].
In the framework of first-principles calculations, we investigate the structural and electronic properties of graphene in contact with as well as sandwiched between WS$_2$ and WSe$_2$ monolayers. We report the modification of the band characteristics
due to the interaction at the interface and demonstrate that the presence of the dichalcogenides results in quantum spin Hall states in the absence of a magnetic field.
The time-dependent density functional based tight-binding (TD-DFTB) approach is generalized to account for fractional occupations. In addition, an on-site correction leads to marked qualitative and quantitative improvements over the original method.
Especially, the known failure of TD-DFTB for the description of sigma -> pi* and n -> pi* excitations is overcome. Benchmark calculations on a large set of organic molecules also indicate a better description of triplet states. The accuracy of the revised TD-DFTB method is found to be similar to first principles TD-DFT calculations at a highly reduced computational cost. As a side issue, we also discuss the generalization of the TD-DFTB method to spin-polarized systems. In contrast to an earlier study [Trani et al., JCTC 7 3304 (2011)], we obtain a formalism that is fully consistent with the use of local exchange-correlation functionals in the ground state DFTB method.
