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## Simple bulk calculations

Obviously, bulk calculations are the easiest calculations that can be performed using VASP.

About which files do you have to worry:

INCAR
POSCAR
POTCAR
KPOINTS

A minimal INCAR file is strongly encouraged: the smaller the INCAR file the smaller the number of possible errors. In general, however, the INCAR file should contain a minimal set of parameters:
SYSTEM = Pd: fcc

ENCUT  = 200.00 eV  # energy cut-off for the calculation
PREC   = Normal     # Normal precision
LREAL  = .FALSE     # real space projection .FALSE. or Auto

ISMEAR =    -5;     # tetrahedron method with Bloechl corrections

We recommend to set the flags mentioned above always for all kind of calculations. If these flags are identical among calculations, then and only then can total energies be compared.

For bulk calculations without internal degrees of freedom, we recommend the tetrahedron method with Blöchl corrections. The method converges rapidly with the number of k-points and requires only minimal interference of the user. It is a good practice to specify the energy cutoffs (ENCUT) manually in the INCAR file, but please always check the POTCAR file (grep ENMAX POTCAR), the maximal ENMAX should correspond to ENCUT and should be set in the INCAR file.

A typical KPOINTS file is shown below:

Gamma centred grid
0
Gamma
11 11 11
0  0  0

The number of k-points and therefore the mesh-size depends on the necessary precision. In most cases, a mesh (leading to a mesh containing approximately 60 points for fcc cells) is sufficient to converge the energy to within meV (see also section 8.6) and might be used as default for bulk calculations. If the system is semiconducting, one can often reduce the grid to points (also read section 8.6). For very accurate calculations (energy differences 1 meV), it might be necessary to increase the number of k-points until convergence of the total energy is reached (for most metals grids of are sufficient).

A typical task performed for bulk materials is the calculation of the equilibrium volume. Unless absolute convergence with respect to the basis set is achieved, volume relaxations using the stress tensor are not recommended and calculations with a constant energy cut-off (CEC) are considered to be preferable to calculations with a constant basis set (CBS) (see section 7.6). For the very same reason, you should not try to obtain the equilibrium volume from calculations that differ in the lattice constant by a few hundreds of an Angstrom. These calculations tend to correspond to CBS calculations (for small changes of the lattice constants the basis set remains usually unchanged). It is preferable to fit the energy over a reasonably large volume range to an equation of states ( in the volume is a good choice). A simple loop over different bulk parameters might be done using a UNIX shell script:

rm WAVECAR
for i in 3.7 3.8 3.9 4.0 4.1
do
cat >POSCAR <<!
fcc:
$i 0.5 0.5 0.0 0.0 0.5 0.5 0.5 0.0 0.5 1 cartesian 0 0 0 ! echo "a=$i" ; vasp
E=tail -1 OSZICAR ; echo $i$E >>SUMMARY.fcc
done
cat SUMMARY.fcc

After executing the batch file, the file SUMMARY.fcc holds the energy for different lattice parameters. The total energy can be fitted to some equation of states to obtain the equilibrium volume, the bulk-modulus etc.

(see also section 8.6) and might be used as Using the script and the parameter files given above a simple energy-volume calculation is possible.

Exercise 1: Perform a simple calculation using the INCAR file given above. Read the OUTCAR-file carefully. Somewhere in the OUTCAR file a set of parameters is written beginning with the line

 SYSTEM =  Pd: fcc

These lines give a complete parameter setting for the job and might be cut from the OUTCAR file and used as a new INCAR file. Go through the lines and figure out, what each parameter means. Using the INCAR and the batch file given above, what is the default setting of ISTART for the first and for all subsequent runs? Is this a convenient setting (constant energy cut-off -- constant basis set)?

Exercise 2: Increase the number of KPOINTS till the total energy is converged to 10 meV. Start with a k-points mesh. Is the equilibrium volume still correct for the k-points mesh? Repeat the calculation for a different smearing (ISMEAR=1). Which choice is reasonable for SIGMA ?

Exercise 3: Calculate the equilibrium lattice constant for different bulk phases (e.g. fcc, sc, bcc) and for different cut-offs ENCUT. The energy differences between different bulk phases (e.g. ) will converge rapidly with the cut-off.

Exercise 4: Calculate the Pulay stress for a specific energy cut-off. Then relax the configuration by setting the Pulay stress explicitly (see section 7.6).

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at