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 KPOINTSA 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 correctionsWe recommend to set the flags mentioned above always for all kind of calculations.

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 0The 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.fccAfter 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: fccThese 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

*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).