Brillouin Zone Sampling in ONETEP

ONETEP is a software package for linear-scaling density-functional theory (DFT) simulations on parallel computers. It uses a localised basis of non-orthogonal Wannier functions (NGWFs) that are optimised in situ. It was designed for large-scale systems with thousands of atoms and, therefore, was engineered under the premise that, for self-consistent calculations, the Brillouin zone is sampled only at its centre, the Gamma-point.

Whilst this is a suitable approximation for many systems, it often leads to the use of larger real-space supercells than is necessary. For many systems of interest, significant computational efficiency could be gained by using a smaller supercell along particular spatial directions in conjunction with denser sampling of the Brillouin zone.

In this work, we introduce two types of Brillouin zone sampling methods in ONETEP. The first is targeted at small periodic systems and exploits the fact that the localisation constraints of the NGWFs can be lifted. The second keeps the NGWFs localised and wavevector (k) dependence is introduced via phase factors in the elements of the Hamiltonian matrix (similar to the tight-binding method). This approach keeps the code linear scaling with k-points and is aimed at intermediate-sized systems. These developments enable much more efficient calculations on short and intermediate-scale periodic systems, including bulk crystals, 2D materials, nanowires, nanotubes, surfaces, and interfaces.