Bloch-state optimal basis sets: An efficient approach for electronic structure interpolation

We present an efficient implementation of the k-space interpolation for electronic structure based on the optimal basis method originally proposed by Shirley [Phys. Rev. B 54, 16464 (1996)] The method allows interpolation onto any k-point from a minimal set of input density functional theory (DFT) wavefunctions. Numerically interpolated eigenvalues have an accuracy within 0.01 eV with very small computational cost. The interpolated wavefunctions were used in the Bethe-Salpeter equation to simulate x-ray absorption spectra of different systems, ranging from small bulk crystals to large intermetallic supercells. The method is extensively tested in terms of verification and accuracy for best practices. The approach is shown to be robust and will greatly help accelerate high-throughput DFT studies.