Self-consistent-charge density-functional tight-binding method for simulations of complex materials properties

TLDR

The standard non‑self‑consistent tight‑binding scheme is equivalent to the zeroth‑order approximation, whereas a second‑order expansion yields a transparent, parameter‑free expression for generalized Hamiltonian matrix elements. The authors extend the tight‑binding approach to improve total energies, forces, and transferability. The method uses a second‑order density‑functional expansion of the Kohn‑Sham energy, self‑consistently redistributes Mulliken charges, and adds a Coulomb term for charge fluctuations to capture long‑range electrostatics and self‑interaction effects. The extension markedly improves transferability.

Abstract

We outline details about an extension of the tight-binding (TB) approach to improve total energies, forces, and transferability. The method is based on a second-order expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations. The zeroth order approach is equivalent to a common standard non-self-consistent (TB) scheme, while at second order a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These 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 additionally includes a Coulomb interaction between charge fluctuations. At large distances this accounts for long-range electrostatic forces between two point charges and approximately includes self-interaction contributions of a given atom if the charges are located at one and the same atom. We apply the new SCC scheme to problems where deficiencies within the non-SCC standard TB approach become obvious. We thus considerably improve transferability.