2 VASP AN INTRODUCTION 15
4. Determine the equilibrium volume (for instance using a quadratic fit of the energy). The equilibrium lattice
constant should be close to 3.526.
5. Now set the equilibrium lattice constant in the POSCAR file and move the ion located at 0.25 0.25 0.25 to 0.24
0.24 0.24, and relax it back to the equilibrium position using VASP. You have to add the lines
NSW = 10 ! allow 10 steps
ISIF = 2 ! relax ions only
IBRION = 2 ! use CG algorithm
to the INCAR file. (At this point you might find it helpful to read section 6.21).
In order to find the minimum, VASP performs a line minimisations of the energy along the direction of the
forces (see 6.21). The line minimisation, requires VASP to take a ”small” trial step into the direction of the
force, then the total energy is re-evaluated. From the energy change and the initial and final forces, VASP
calculates the position of the minimum. For carbon, the automatically chosen trial step is much too large,
and VASP can run more efficiently, if the parameter POTIM is set in the INCAR file:
POTIM = 0.1 ! reduce trial step
Do that and start once again from a more exited structure (i.e. 0.20,0.20,0.20).
At the end of any job, VASP writes the final positions to the file CONTCAR. This file has the same format as
the POSCAR file, and it is possible to continue a run, by copying CONTCAR to POSCAR and running VASP again.
6. As a final exercise, change the lattice constant in the POSCAR file to 3.40, and change ISIF in the INCAR file
to
ISIF = 3 ! relax ions + volume
POTIM = 0.1 ! you need to specify POTIM as well
and start once again. If ISIF is set to 3, VASP relaxes the ionic positions and the cell volume.
Do not forget to check the OUTCAR file from time to time.
7. The final lattice constant will be quite accurate (around 3.510
˚
A). The small difference to the lattice constant
obtained by fitting the energy volume curve is due to the Pulay stress (see section 7.6): the stress tensor is
only correct if the calculations are fully converged with respect to the basis set. There are several possibilities
to solve this problem:
8. Increase the plane wave cutoff by 30% with respect to the standard value in the INCAR file (ENMAX=550). Now
the basis set is almost converged, and more accurate results for the lattice constant can be obtained. Try this
for carbon, and increase the accuracy of the electronic ground-state calculation by setting
EDIFF = 1E-7 ! very high accuracy required 10^-7 eV
in the INCAR file. Start from the CONTCAR file of the last calculation (i.e. copy CONTCAR to POSCAR).
9. The Pulay error is independent of the structure, so it can be evaluated once and for ever using first a large
basis-set and than a small one. Start at the equilibrium structure, with a high cutoff (ENCUT=550). The stress
tensor should be zero.
Then use the default cutoff. The stress is now -43 kBar. This yields an estimation of the possible errors caused
by the basis set incompleteness. (You might correct the relaxation by setting
PSTRESS = -43 ! Pulay stress = -43 kB
in the INCAR file, but it is usually preferable to increase ENCUT).
Hopefully this small example has given you an idea how VASP works.