The density-functional tight-binding (DFTB) method is an approximation to traditional density functional theory (DFT) , roughly 2-3 orders of magnitude faster yet providing detailed electronic structure by solving Kohn-Sham equations using a parameterized Hamiltonian. DFTB methods can be employed in simulations of processes that involve chemical reactions, electron excitation, electron transfer, and mass and ion transport for systems containing several tens of thousands of atoms. Linear scaling algorithms exist and have been developed and applied to systems as large as 100 million atoms .