A Self-Consistent Charge Density-Functional Based Tight-Binding Method for Predictive 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 transferability of more sophisticated density-functional and post-Hartree-Fock methods with the aim to perform highly predictive materials simulations of technological relevant sizes in physics, chemistry and biology. Following Harris, Foulkes and Haydock, the methods are based on an expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations at a given reference density. While the zeroth order approach is equivalent to a common standard non-self-consistent tight-binding (TB) scheme, at second order by variationally treating the approximate Kohn-Sham energy a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These matrix elements are modified by a Self-Consistent redistribution of Mulliken Charges (SCC). Besides the usual “band-structure” and short-range repulsive terms the final approximate Kohn-Sham energy explicitly includes Coulomb interaction between charge fluctuations. The new SCC-scheme is shown to successfully apply to problems, where defficiencies within the non-SCC standard TB-approach become obvious. These cover defect calculations and surface studies in polar semiconductors (see M. Haugk et al. of this special issue), spectroscopic studies of organic light-emitting thin films, briefly outlined in the present article, and atomistic investigations of biomolecules (see M. Elstner et al. of this special issue).