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Planewaves

This page gives hints on how to perform numerically precise calculations with planewaves or projector- augmented waves and pseudopotentials with the ABINIT package.

Introduction

The numerical precision of the calculations depends on many settings, among which the definition of a basis set is likely the most important. With planewaves, there is one single parameter, ecut that governs the completeness of the basis set.

The wavefunction, density, potentials are represented in both reciprocal space (plane waves) and real space, on a homogeneous grid of points. The transformation from reciprocal space to real space and vice-versa is made thanks to the Fast Fourier Transform (FFT) algorithm. With norm-conserving pseudopotential, ecut is also the main parameter to define the real space FFT grid, In PAW, the sampling for such quantities is governed by a more independent variable, pawecutdg. More precise tuning might be done by using boxcutmin and ngfft.

Avoiding discontinuity issues with changing the size of the planewave basis set is made possible thanks to ecutsm.

The accuracy variable enables to tune the accuracy of a calculation by setting automatically up to seventeen variables.

Many more parameters govern a PAW computation than a norm-conserving pseudopotential calculation. They are described in a specific page topic_PAW. For the settings related to wavelets, see topic_Wavelets.

compulsory:

  • ecut Energy CUToff
  • pawecutdg PAW - Energy CUToff for the Double Grid

basic:

useful:

  • boxcutmin BOX CUT-off MINimum
  • ngfft Number of Grid points for Fast Fourier Transform

expert:

  • mqgrid Maximum number of Q-space GRID points for pseudopotentials
  • nc_xccc_gspace Norm-Conserving pseudopotentials - use XC Core-Correction in G-SPACE

internal:

  • %mgfft Maximum of nGFFT
  • %mgfftdg Maximum of nGFFT for the Double Grid
  • %mpw Maximum number of Plane Waves
  • %nfft Number of FFT points
  • %nfftdg Number of FFT points for the Double Grid

Selected Input Files

No input file associated to this topic.

Tutorials

  • 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 first lesson on the the projector-augmented wave technique.