This page gives hints on how to to specify bands and occupation numbers, for metals or insulators with the ABINIT package.
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 1 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 (Verstraete scheme) using tphysel.
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.
Related Input Variables¶
- nband Number of BANDs
- occ OCCupation numbers
- occopt OCCupation OPTion
- tsmear Temperature of SMEARing
- fband Factor for the number of BANDs
- nbdbuf Number of BanDs for the BUFfer
- tphysel Temperature (PHYSical) of the ELectrons
Selected Input Files¶
- The lesson 1 deals with the H2 molecule: get the total energy, the electronic energies, the charge density, the bond length, the atomisation energy
- The lesson 2 deals again with the H2 molecule: convergence studies, LDA versus GGA
- The lesson 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 lesson 4 deals with crystalline aluminum (a metal), and its surface: occupation numbers, smearing the Fermi-Dirac distribution, the surface energy, and again, convergence studies …