This is a very common questions from people who start to do calculations with plane wave codes. There are two reasons why the energy vs. volume plot looks jagged:

- Basis set incompleteness.
The basis set is discrete and incomplete, and when the volume changes,
additional plane waves are added. That causes small discontinuous changes
in the energy.

Solutions:

- use a larger plane wave cutoff:

This is usually the preferred and cheapest solution. - use more k-points :

This solves the problem, because the criterion for including a plane wave in the basis set is:

- use a larger plane wave cutoff:
- However the most probable reason for the jagged E(V) curve
is another one:
For
`PREC=High`the FFT grids are chosen so that is exactly evaluated. For`PREC=Med`the FFT grids are set to 3/4 of the value that is in principle required for an exact evaluation of . This introduces small errors, because when the volume changes the FFT grids do change discontinuously. In other words, at each volume a different FFT-grid is used, causing the energy to jump discontinuously.

Solutions:- Set your FFT grids manually. Choose that one that is used per default for the largest volume
- use PREC=High. In the new version (starting from VASP.4.4.3) this
also increases the plane wave cutoff by 30 %.
If this is undesirable, the plane wave cutoff can be fixed
manually by specifying
`ENMAX=...`in the INCAR file

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