This page gives hints on how to set parameters related to the electronic wavevectors (k-points) with the ABINIT package.
Since ABINIT is based on periodic boundary conditions, every wavefunction is characterized by a wavevector, usually denoted k-point. Any list of k-point can be specified, thanks to the keywords nkpt and kpt.
Still, k-points are used in two different contexts in the vast majority of cases:
- the sampling of the Brillouin Zone, with the goal to produce integrated quantities (e.g. the charge density, the electronic energy, the electronic DOS …) that are numerically precise;
- or the specific computation of wavefunctions and eigenenergies e.g. to get an electronic band structure.
In the first case, the Brillouin zone must be sampled adequately, with grids that, in general will be homogeneous distributions of k-points throughout the Brillouin Zone (e.g. Monkhorst-Pack grids, or their generalisations). For such grids, see ngkpt, nshiftk, shiftk or even the more general kptrlatt. A list of interesting k point sets can be generated automatically, including a measure of their accuracy in term of integration within the Brillouin Zone, see prtkpt, kptrlen. For metals, a joint convergence study on tsmear and the k-point grid is important.
For the definition of a path of k-points, see topic_ElecBandStructure.
More detailed explanation concerning the convergence with respect to the k-point sampling. The number of k-points to be used for this sampling, in the full Brillouin zone, is inversely proportional to the unit cell volume, but may also vary a lot from system to system. As a rule of thumb, a system with a large band gap will need few k-points, while metals will need lot of k-points to produce converged results. For large systems, the inverse scale with respect to the unit cell volume is unfortunately stopped because at least one k-point must be used. The effective number of k-points to be used will be strongly influenced by the symmetries of the system, since only the irreducible part of the Brillouin zone must be sampled. Moreover the time-reversal symmetry (k equivalent to -k) can be used for ground-state calculations, to reduce sometimes even further the portion of the Brillouin zone to be sampled. The number of k points to be used in a calculation is named nkpt.
There is another way to take advantage of the time-reversal symmetry, in the specific case of k-points that are invariant under k => -k , or are sent to another vector distant of the original one by some vector of the reciprocal lattice. See below for more explanation about the advantages of using these k-points.
As a rule of thumb, for homogeneous systems, a reasonable accuracy may be reached when the product of the number of atoms by the number of k-points in the full Brillouin zone is on the order of 50 or larger, for wide gap insulators, on the order of 250 for small gap semiconductors like Si, and beyond 500 for metals, depending on the value of the input variable tsmear. As soon as there is some vacuum in the system, the product natom * nkpt can be much smaller than this (for an isolated molecule in a sufficiently large supercell, one k-point is enough).
Related Input Variables¶
- chksymbreak CHecK SYMmetry BREAKing
- kptopt KPoinTs OPTion
- ngkpt Number of Grid points for K PoinTs generation
- istwfk Integer for choice of STorage of WaveFunction at each k point
- kpt K - PoinTs
- kptbounds K PoinT BOUNDarieS
- kptnrm K - PoinTs NoRMalization
- kptrlatt K - PoinTs grid: Real space LATTice
- kptrlen K - PoinTs grid: Real space LENgth
- ndivk Number of DIVisions of K lines
- ndivsm Number of DIVisions for the SMallest segment
- nkpath Number of K-points defining the PATH
- nkpt Number of K - Points
- nshiftk Number of SHIFTs for K point grids
- prtkpt PRinT the K-PoinTs sets
- shiftk SHIFT for K points
- wtk WeighTs for K points
- %kptns K-PoinTs re-Normalized and Shifted
- %kptns_hf K-PoinTs re-Normalized and Shifted, for the Hartree-Fock operator
Selected Input Files¶
- 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 …