Groundstate calculation of silver (Lithium version)

Units in exciting

By default, all quantities in the exciting code are given in atomic units: Energies in Hartree, lengths in Bohr, etc. (see Input Reference). In case other units are desirable, they can be converted using templates as a post-processing to exciting's standard output.

Groundstate calculation of silver

 

The first step of any density-functional calculation is the determination of the groundstate total energy and electron density.

The starting point of a groundstate calculation is the crystal structure, only. At the beginning of a groundstate calculation, an initial electron density is generated, which is obtained from a superposition of atomic densities. Thus, this initial electron density lacks electron-electron and electron-ion interactions between atoms and is normally a rather crude approximation of the density.

Then, the calculation iteratively goes through the following steps:

1. Determine the potential from the electron density.
2. Solve the Kohn-Sham (KS) equations to get the eigenfunctions and eigenvalues as well as the total energy.
3. Calculate the electron density from the KS eigenfunctions.
4. Create a new charge density, mixing the electron density from the current iteration with the ones of previous iteration (to ensure a good convergence behavior).
5. Start again with (1).

Such a sequence of steps is usually called an iteration. The code will repeat such iterations, until the potential (or total energy, or charge density, …) obtained at the end of the last iteration is consistent with the one of the previous iteration. Thus, this kind of calculations is often called self-consistent field (SCF) calculation, and an iteration is often referred to as an SCF cycle.

To prepare your calculation, create a new, empty directory named Ag somewhere on your filesystem. In this directory, save the following lines as input.xml.

<input>
 
  <title>Ag</title>
 
  <structure speciespath="$EXCITINGROOT/species">
    <crystal scale="7.7201">
      <basevect>0.5 0.5 0.0</basevect>
      <basevect>0.5 0.0 0.5</basevect>
      <basevect>0.0 0.5 0.5</basevect>
    </crystal>
    <species speciesfile="Ag.xml" chemicalSymbol="Ag">
      <atom coord="0.0  0.0  0.0" />
    </species>
  </structure>
 
  <groundstate ngridk="8  8  8"></groundstate>
 
</input>

N.B.: Do not forget to replace in the input.xml the string "$EXCITINGROOT" by the actual value of the environment variable $EXCITINGROOT using the command

$ SETUP-excitingroot.sh

If the visualization program XCrySDen is set up appropriately (find here how to do this: XCrySDen Setup for exciting) you can visualize the structure in the exciting input file executing

$ xcrysden --exciting input.xml

After this, start the groundstate calculation by executing the following command in the Ag directory:

$ excitingser

The calculation should roughly take 1 minute. During the calculation, output files are created, which contain all kind of information on your material system and on the calculation. Some of the output files are already created at the beginning of the calculation and will not be changed anymore during the run. The most important among them are:

filename description LATTICE.OUT Information on the lattice: Primitive lattice vectors, unit cell volume, reciprocal lattice vectors, etc. SYMCRYS.OUT Information on the symmetry operations of the crystal; more symmetry information is found in the files SYMT2.OUT, SYMSITE.OUT, SYMMULT_TABLE.OUT, SYMMULT.OUT, SYMLAT.OUT, SYMINV.OUT, and SYMGENR.OUT. KPOINTS.OUT List of k-points, their coordinates (in units of the reciprocal lattice vectors), weights, matrix size. IADIST.OUT Interatomic distances; useful to check the correctness of an input file. EQATOMS.OUT Information on equivalency of atoms due to the crystal symmetry. geometry.OUT.xml Structural information on your system. This will often be identical to the element structure in your input file, but may differ in case you set the structure attribute primcell = "true".

Other files are updated or extended in each iteration and contain information about the scf calculation. Here are the most important ones:

filename description info.xml Master output file in XML format, containing the essential information on the material system, on the parameters of the calculation, on the results (total energy, energy contributions, charge contributions, Fermi energy …) of each iteration, and more … INFO.OUT Extended version of info.xml, displayed as plain text file. TOTENERGY.OUT Total energy (Hartree); each line corresponds to one iteration. EFERMI.OUT Fermi energy (Hartree). FERMIDOS.OUT Density of states at the Fermi level (states/unit cell/Hartree); each line corresponds to one iteration. EIGVAL.OUT Eigenvalues (energies) of the valence bands, for each k-point and band. EVALCORE.OUT Eigenvalues (=energy levels) of the core states. LINENGY.OUT Linearization energies as fixed in the species files (if searchE="false" for the corresponding linearisation energy in the species.xml file) or determined by exciting (if searchE="true" for the corresponding linearisation energy in the species.xml file).

Exercises

With the table above, find out the following properties of your calculation — to do so, find out first in which output files they are contained:

• How many iterations did the calculation go through?
• What is the total energy for the first iteration (which started from the superposition of atomic electron densities), and of the converged calculation (last iteration)?
• What was the change in total energy between the
  1. first two iterations?
  2. last two iterations?
• What is the Fermi energy of the system?
• How many occupied valence bands are there in the system?
• How much charge is there inside the muffin-tin sphere, and how much is found in the interstitial region?

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