# Building an input file

### This page gives hints on how to build an input file for a ground state calculation with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (FJ)

Mentioned in help_features#2.

### Table of content:

- 1. Introduction.
- 2. Example(s)
- 3. Related lesson(s) of the tutorial.

### 1. Introduction.

The computation of the ground state energy of an assembly of nuclei and electrons placed in a repeated cell can be done using (1) plane waves and norm-conserving pseudopotentials, or, (2) so-called "Projector-Augmented Waves" (PAW method), with appropriate pseudoatomic data, or (3) wavelets. The wavelet framework is described here.
In the plane wave framework, the program admits many different types of pseudopotentials. There are several complete sets of norm-conserving pseudopotentials available for most elements of the periodic table. The recommended one (GGA) comes from the ONCVPSP generator (with spin-orbit coupling). For PAW calculation,the recommended one (GGA and LDA) is the JTH table in the PAW XML format. The choice between norm-conserving pseudopotentials or PAW is deduced automatically by the choice of the pseudopotential in the "files" file. An input file must specify the following items:

- the crystalline structure and symmetries.
- the set of k-points used.
- the exchange and correlation functional.
- convergency settings .
- possibly, PAW special settings.
- possibly, input variables for spin-polarized systems and spin orbit coupling calculations.
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### 2. Example(s)

An example of a minimal input file to calculate the ground state of crystalline aluminium is given here:

     # Crystalline aluminum. Calculation of the total energy
     # at fixed number of k points and broadening.

     #Definition of occupation numbers
     occopt 4
     tsmear 0.05

     #Definition of the unit cell
     acell 3*7.60           # This is equivalent to   7.60 7.60 7.60
     rprim  0.0  0.5  0.5   # FCC primitive vectors (to be scaled by acell)
            0.5  0.0  0.5
            0.5  0.5  0.0

     #Definition of the atom types
     ntypat 1          # There is only one type of atom
     znucl 13          # The keyword "znucl" refers to the atomic number of the
                       # possible type(s) of atom. The pseudopotential(s)
                       # mentioned in the "files" file must correspond
                       # to the type(s) of atom. Here, the only type is Aluminum

     #Definition of the atoms
     natom 1           # There is only one atom per cell
     typat 1           # This atom is of type 1, that is, Aluminum
     xred  0.0  0.0  0.0 # This keyword indicate that the location of the atoms
                         # will follow, one triplet of number for each atom
                         # Triplet giving the REDUCED coordinate of atom 1.

     #Definition of the planewave basis set
     ecut  6.0         # Maximal kinetic energy cut-off, in Hartree
     pawecutdg  10.0   #Maximal kinetic energy cut-off, in Hartree for the fine grid in case of PAW calculation

     #Definition of the k-point grid
     ngkpt 2 2 2       # This is a 2x2x2 FCC grid, based on the primitive vectors
     chksymbreak 0
     #Definition of the SCF procedure
     nstep 10          # Maximal number of SCF cycles
     toldfe 1.0d-6     # Will stop when, twice in a row, the difference
                       # between two consecutive evaluations of total energy
                       # differ by less than toldfe (in Hartree)
                       # This value is way too large for most realistic studies of materials

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### 3. 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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