mirror of https://github.com/abinit/abinit.git
52 lines
2.2 KiB
Markdown
52 lines
2.2 KiB
Markdown
---
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description: How to perform numerically precise calculations with planewaves or projector- augmented waves and pseudopotentials
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authors: FJ
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---
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<!--- This is the source file for this topics. Can be edited. -->
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This page gives hints on how to perform numerically precise calculations with planewaves or projector-
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augmented waves and pseudopotentials with the ABINIT package.
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## Introduction
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The numerical precision of the calculations depends on many settings, among
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which the definition of a basis set is likely the most important. With
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planewaves, there is one single parameter, [[ecut]] that governs the
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completeness of the basis set.
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The wavefunction, density, potentials are represented in both reciprocal space
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(plane waves) and real space, on a homogeneous grid of points. The
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transformation from reciprocal space to real space and vice-versa is made
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thanks to the Fast Fourier Transform (FFT) algorithm. With norm-conserving
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pseudopotential, [[ecut]] is also the main parameter to define the real space
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FFT grid, In PAW, the sampling for such quantities is governed by a more
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independent variable, [[pawecutdg]]. More precise tuning might be done by
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using [[boxcutmin]] and [[ngfft]].
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Avoiding discontinuity issues with changing the size of the planewave basis
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set is made possible thanks to [[ecutsm]].
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The [[accuracy]] variable enables to tune the accuracy of a calculation by
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setting automatically up to seventeen variables.
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Many more parameters govern a PAW computation than a norm-conserving
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pseudopotential calculation. They are described in a specific page
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[[topic:PAW]]. For the settings related to wavelets, see [[topic:Wavelets]].
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## Related Input Variables
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{{ related_variables }}
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## Selected Input Files
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{{ selected_input_files }}
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## Tutorials
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* [[tutorial:base2|The tutorial 2]] deals again with the H2 molecule: convergence studies, LDA versus GGA
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* [[tutorial:base3|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 ...
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* The first tutorial on the [[tutorial:paw1|the projector-augmented wave]] technique.
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