ISMEAR

ISMEAR is a parameter used in electronic structure calculations, most notably in VASP, to choose the numerical scheme for broadening electronic occupancies near the Fermi level. Smearing is applied to improve convergence of the self-consistent field iterations and to facilitate Brillouin-zone integration in systems with partial occupancies, especially metals. The selected smearing method interacts with the smearing width, specified by SIGMA, which is given in electron volts.

Common options for ISMEAR include Gaussian broadening (ISMEAR = 0), Methfessel-Paxton schemes (ISMEAR = 1 or 2), and the tetrahedron method with Blöchl corrections (negative values, such as ISMEAR = -5). The choice of method can influence calculated total energies, forces, and densities of states, and thus results should be interpreted with the smearing scheme in mind. For insulators and semiconductors, small or no smearing is often appropriate, whereas metals typically require one of the smearing schemes to achieve stable convergence. The exact values of SIGMA determine the effective broadening and should be chosen in relation to the material system and desired accuracy.

Practically, ISMEAR and SIGMA are used to balance convergence behavior with accuracy. Researchers may compare results across several smearing schemes to assess sensitivity, and it is important to report the smearing method and width when presenting calculated properties.