DFTB2

DFTB2, short for density functional tight binding version 2, is a semi-empirical quantum mechanical method used to efficiently estimate electronic structure and total energies of molecules and condensed-phase systems. It is derived from density functional theory by expanding the Kohn–Sham energy around a reference electron density and truncating the expansion after second order. The second-order truncation gives the DFTB2 variant. There are related forms such as DFTB0 (zero order) and DFTB3 (third order), with SCC-DFTB referring to the self-consistent charge formulation that includes charge fluctuations.

The method uses a minimal basis set of atomic orbitals and a two-center approximation for Hamiltonian and overlap matrices. Parameters are obtained from reference DFT calculations or fitted to reproduce certain properties, and a short-range repulsive potential is added to capture remaining interactions. In DFTB2, the total energy comprises a band-structure-like term, a self-consistent charge correction that accounts for charge transfer through a Coulomb-like interaction, and a repulsive energy term.

DFTB2 is designed to be computationally efficient, enabling rapid geometry optimization, vibrational analysis, and large-scale molecular dynamics for systems containing hundreds to tens of thousands of atoms. It is widely used in materials science, chemistry, and biology where full DFT would be too costly. The accuracy of DFTB2 depends on the quality of the parameter set and is generally suitable for systems well described by the reference density; it may be less reliable for unusual bonding, strong correlation, dispersion-dominated interactions unless dispersion corrections are included, or excited-state properties.

Software implementations, such as DFTB+, provide DFTB2, along with related variants, to support research in electronic structure and dynamics.