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BandOcc

This page gives hints on how to specify bands and occupation numbers, for metals or insulators with the ABINIT package.

Introduction

Metallic as well as insulating systems can be treated, depending on the value of occopt. The default value of occopt corresponds to an insulator (or finite molecule): the number of bands (or states for a molecule) is deduced from the number of electrons brought by each pseudopotential ion, and then all the bands are occupied (by two electrons in case of a non-spin-polarized system, or by one electron in the case of a spin-polarized system), and a small number of empty bands are added, e.g. to obtain the band gap.

For a metallic system, use a value of occopt between 3 and 7. ABINIT will compute a default number of bands, including some nearly unoccupied ones, and find the occupation numbers. The different values of occopt correspond to different smearing schemes (smearning defined by tsmear for defining the occupation numbers, e.g. Fermi broadening, the Gaussian broadening, the Gaussian-Hermite broadening, as well as the modifications proposed by Marzari. Finite temperatures can also be treated thanks to a smearing scheme ([Verstraete2002] scheme) using tphysel.

For gapped materials, the treatment of quasi-Fermi energies in the valence and conduction band, with populations of electrons (in the conduction bands) and holes (in the valence bands) is available with occopt=9.

It is possible to define manually the number of bands (input variable nband) as well as the occupation numbers (input variable occ). This might be useful to perform a Δ-SCF calculation for excited states.

basic:

  • nband Number of BANDs
  • occ OCCupation numbers
  • occopt OCCupation OPTion
  • tsmear Temperature of SMEARing

useful:

  • fband Factor for the number of BANDs
  • ivalence Index of the highest VALENCE band
  • nbdbuf Number of BanDs for the BUFfer
  • nqfd Number of Quasi Fermi-Dirac excited carriers
  • tphysel Temperature (PHYSical) of the ELectrons

internal:

Selected Input Files

v1:

v2:

Tutorials

  • The tutorial 1 deals with the H2 molecule: get the total energy, the electronic energies, the charge density, the bond length, the atomisation energy
  • The tutorial 2 deals again with the H2 molecule: convergence studies, LDA versus GGA
  • The tutorial 3 deals with crystalline silicon (an insulator): the definition of a k-point grid, the smearing of the cut-off energy, the computation of a band structure, and again, convergence studies …
  • The tutorial 4 deals with crystalline aluminum (a metal), and its surface: occupation numbers, smearing the Fermi-Dirac distribution, the surface energy, and again, convergence studies …