Fermi energy determination for advanced smearing techniques

Smearing techniques are widely used in first-principles calculations of metallic and magnetic materials where they improve the accuracy of Brillouin-zone sampling and lessen the impact of level-crossing instabilities. Smearing introduces a fictitious electronic temperature that smooths the discontinuities of the integrands; consequently, a corresponding fictitious entropic term arises, and needs to be considered in the total freeenergy functional. Advanced smearing techniques-such as Methfessel-Paxton and cold smearing-have been introduced to guarantee that the system's total free energy remains independent of the smearing temperature, at least, up to the second order. In doing so, they give rise to nonmonotonic occupation functions (and, for Methfessel-Paxton, nonpositive definite), which can result in the chemical potential not being uniquely defined. We explore this shortcoming in detail and introduce a numerical protocol utilizing Newton's minimization method that is able to identify the desired Fermi energy. We validate the method by calculating the Fermi energy of similar to 20 000 materials and comparing it with the results of standard bisection approaches. In passing, we also highlight how traditional approaches, based on Fermi-Dirac or Gaussian smearing, are actually equivalent for all practical purposes, provided the smearing width is appropriately renormalized by a factor of similar to 2.565.

Automated summary

Smearing techniques are used to improve the accuracy of Brillouin-zone sampling and reduce level-crossing instabilities in first-principles calculations of metallic and magnetic materials. Advanced smearing techniques, like Methfessel-Paxton and cold smearing, ensure that the total free energy of the system remains independent of the smearing temperature. However, nonmonotonic occupation functions and the lack of unique definition for the chemical potential can be drawbacks. To address this, a numerical protocol using Newton's minimization method is introduced to identify the desired Fermi energy.