  This article is a pedagogical introduction to density-functional
tight-binding (DFTB) method. We derive it from the density-functional theory,
give the details behind the tight-binding formalism, and give practical recipes
for parametrization: how to calculate pseudo-atomic orbitals and matrix
elements, and especially how to systematically fit the short-range repulsions.
Our scope is neither to provide a historical review nor to make performance
comparisons, but to give beginner's guide for this approximate, but in many
ways invaluable, electronic structure simulation method--now freely available
as an open-source software package, hotbit.
