Accurate bulk relaxations with internal parameters (two)

It is possible to avoid volume relaxation in many cases: The method we have used quite often in the past, is to relax the structure (cell shape and internal parameters) for a set of fixed volumes (ISIF=4). The final equilibrium volume and the groundstate energy can be obtained by a fit to an equation of state. The reason why this method is better than volume relaxation is that the Pulay stress is almost isotropic, and thus adds only a constant value to the diagonal elements of the stress tensor. Therefore, the relaxation for a fixed volume will give an almost correct structure.

The outline for such a calculation is almost the same as in the previous section. But in this case, one has to do the calculations for a set of fixed volumes. At first sight this seems to be much more expensive than method number one (outlined in the previous section). But in many cases the additional costs are only small, because the internal parameters do not change very much from volume to volume.

1. Select one volume and relax from starting structure keeping the volume fixed (ISIF=4 see sec. 6.23; ISMEAR=0 or 1, see section 6.37).
2. Start a second relaxation from previous CONTCAR file (if the initial cell shape was reasonable this step can be skipped, if the cell shape is kept fixed, you never have run VASP twice).
3. As a final step perform one more energy calculation with the tetrahedron method switched on (ISMEAR=-5), to get very accurate unambiguous energies (no relaxation for the final run).

The method has also other advantages, for instance the bulk modulus is readily available. We have found in the past that this method can be used safely with the default cutoff. (see also section 9.2).