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11.1 Simple bulk calculations


Bulk calculations are the easiest calculations which can be performed using VASP.

About which files do you have to worry:
A minimal INCAR file is strongly encouraged: the smaller the INCAR file the smaller the number of possible errors. In general the INCAR file might look like:
SYSTEM = Pd: fcc

 Electronic minimisation
   ENCUT  = 200.00 eV  ! energy cut-off for the calculation (optional)

 DOS related values
   ISMEAR =    -5;     ! tetrahedron method with Bloechel corrections
For bulk calculation without internal degrees of freedom we recommend the tetrahedron method with Bloechel corrections. The method converges rapidly with the number of k-points and requires only minimal interference of the user.

If your cell contains only one atomic species the file will be similar to (use the makeparam utility to create this file, before running makeparam be sure that you POSCAR file corresponds to the most expanded volume):

C-----General parameters always needed ...

C-----Parameter for non-local contribution
The NGX,Y,Z setting given above will be sufficient even for relatively accurate calculations, the augmentation part (NGXF...) will be also sufficient in most cases. With this file it is possible to use reciprocal and the real space projectors (for reasons of efficiency only reciprocal projectors should be used for such a small cell).

The KPOINTS file might have the following contents:

Monkhorst Pack
Monkhorst Pack
 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 tex2html_wrap_inline5091 mesh (leading to a mesh containing approximately 60 points) is sufficient to converge the energy to within 10 meV (see also section 10.6), and might be used as some kind of default for bulk calculations. If the system is semiconducting you can reduce the mesh to tex2html_wrap_inline5111 (also read section 10.6). For very accurate calculations (energy differences 1 meV), it is necessary to increase the number of k-points continuously and to check when the band-structure energy is converged (in most cases a mesh of tex2html_wrap_inline5113 is 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 relaxation's 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 9.6). Due to the same reason you should not try to obtain the equilibrium volume from calculations which differ in the lattice constant by a few hundreds of an Angstrom. These calculations tend to be CBS calculation and not a CEC calculation (for a very small change in the lattice constant the basis set will remain unchanged). It is preferable to fit the energy over a certain energy range to a equation of states. A simple loop over different bulk parameters might be done using a UNIX shell script:

for i in 3.7 3.8 3.9 4.0 4.1
cat >POSCAR <<!
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
0 0 0
echo "a= $i" ; vamp
E=`tail -1 OSZICAR` ; echo $i $E >>SUMMARY.fcc
cat SUMMARY.fcc
After a run the file SUMMARY.fcc contains 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 and so on. 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 following runs? Is this a convenient setting (constant energy cut-off -- constant basis set)?

Exercise 2: Set all values in the file to their minimum values, recompile VASP, and start the calculation once again.

Exercise 3: Increase the number of KPOINTS till the total energy is converged to 10 meV. Start with a tex2html_wrap_inline5111 k-points mesh. Is the equilibrium volume still correct for the tex2html_wrap_inline5111 k-points mesh? Repeat the calculation for a different smearing (ISMEAR=1). What is a good choice for SIGMA?

Exercise 4: 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. tex2html_wrap_inline5119 ) will converge rapidly with the cut-off.

Exercise 5: Calculate the Pulay stress for a specific energy cut-off. Then relax the configuration by setting the Pulay stress explicitly (see section 9.6). The dynamic calculation requires the setting of

Use the conjugate-gradient algorithm.

next up previous contents
Next: 11.2 Bulk calculations with Up: 11 Examples Previous: 11 Examples

Mon Mar 29 10:38:29 MEST 1999