Enhanced Twist-Averaging Technique for Magnetic Metals: Applications using Quantum Monte Carlo

We propose an improved twist-averaging scheme for quantum Monte Carlo methods that use converged Kohn-Sham or Hartree-Fock orbitals as the reference. This twist-averaging technique is tailored to sample the Brillouin zone of magnetic metals, although it naturally extends to nonmagnetic conducting systems. The proposed scheme aims to reproduce the reference magnetization and achieves charge neutrality by construction, thus avoiding the large energy fluctuations and the postprocessing needed to correct the energies. It shows the most robust convergence of total energy and magnetism to the thermodynamic limit when compared to four other twist-averaging schemes. Diffusion Monte Carlo applications are shown on nonmagnetic Al and ferromagnetic $\alpha$-Fe. The cohesive energy of Al in the thermodynamic limit shows an excellent agreement with the experimental result. Furthermore, the magnetic moments in $\alpha$-Fe exhibit rapid convergence with an increasing number of twists.