Bands and occupation numbers for metals and insulators
This page gives hints on how to to specify bands and occupation numbers, for metals or insulators with the ABINIT package.
Copyright (C) 2016-2017 ABINIT group (FJ)
Mentioned in topic_k-points, help_features#2.2.
Table of content:
1. 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 1 electron in the cas 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.
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2. Related lesson(s) of the tutorial.
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 ...
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3. Related input variables.
Basic input variables:
... nband [Number of BANDs]
... occ [OCCupation numbers]
... occopt [OCCupation OPTion]
... tsmear [Temperature of SMEARing]
Useful input variables:
... fband [Factor for the number of BANDs]
... nbdbuf [Number of BanDs for the BUFfer]
... tphysel [Temperature (PHYSical) of the ELectrons]
Relevant internal variables:
... %mband [Maximum number of BANDs]
... %nelect [Number of ELECTrons]
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4. Selected input files.
WARNING : as of ABINITv8.6.x, the list of input files provided in the specific section of the topics Web pages is still to be reviewed/tuned. In some cases, it will be adequate, and in other cases, it might be incomplete, or perhaps even useless.
The user can find some related example input files in the ABINIT package in the directory /tests, or on the Web:
tests/v1/Input: t02.in
t04.in
tests/v2/Input: t45.in
t46.in
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