A self-consistent charge density-functional based tight-binding method forpredictive materials simulations in physics, chemistry and biology

We outline recent developments in quantum mechanical atomistic modelling of complex materials properties that combine the efficiency of semi-empirical quantum-chemistry and tight-binding approaches with the accuracy and trans ferability of more sophisticated density-functional and post-Hartree-Fock m ethods with the aim to perform highly predictive materials simulations of t echnological relevant sizes in physics, chemistry and biology. Following Ha rris, Foulkes and Haydock, the methods are based on an expansion of the Koh n-Sham total energy in density-functional theory (DFT) with respect to char ge density fluctuations at a given reference density. While the zeroth orde r approach is equivalent to a common standard non-self-consistent tight-bin ding (TB) scheme, at second order by variationally treating the approximate Kohn-Sham energy a transparent, parameter-free, and readily calculable exp ression for generalized Hamiltonian matrix elements may be derived. These m atrix elements are modified by a Self-Consistent redistribution of Mulliken Charges (SCC). Besides the usual "band-structure" and short-range repulsiv e terms the final approximate Kohn-Sham energy explicitly includes Coulomb interaction between charge fluctuations. The new SCC-scheme is shown to suc cessfully apply to problems, where defficiencies within the non-SCC standar d TB-approach become obvious. These cover defect calculations and surface s tudies in polar semiconductors (see M. Haugk et al. of this special issue), spectroscopic studies of organic light-emitting thin films, briefly outlin ed in the present article, and atomistic investigations of biomolecules (se e M. Elstner et al. of this special issue).