9.6.2 Accurate bulk relaxations with internal parameters (one)

 

The general message is: whenever possible avoid volume relaxation with the default energy cutoff. Either increase the basis set by setting ENCUT manually in the INCAR file, or use the method suggested below, which avoids volume relaxations at all. If volume relaxation are the only possible and feasible option please use the following step by step procedure (which minimize errors to a minimum):

1. Relax from starting structure (ISMEAR should be 0 or 1, see section 7.33).
2. Start a second relaxation from previous CONTCAR file (re-relaxation).
3. As a final step perform one more energy calculation with the tetrahedron method switched on (i.e. ISMEAR=-5), to get very accurate and unambiguous energies (no relaxation for the final run). The final calculation should be done with PREC=High, to get very accurate energies.

A few things should be remarked here: Never take the energy obtained at the end of a relaxation run, if you allow for cell shape relaxations (because the final basis set might not be isotropic). The relaxation will give a structure which is correct to first order, the final errors in step 3 is of second order (with respect to the structural errors). If you take the energy directly from the relaxation run errors are usually significantly larger. Another important point is that the most accurate results for the relaxation will be obtained if the starting cell parameters are very close to the final cell parameters. If different runs yield different results, then the run which started from the configuration which was closest to the relaxed structure, is probably most reliable.

We strongly recommend to do any volume (and to lesser extend cell shape) relaxation with an increased basis set. ENCUT=1.3 default cutoff is reasonable accurate in most cases. If the default cutoff is used for the relaxation the Pulay stress corrections outlined below should be used (but we can not really recommend this technique). Also mind, that one is often saved by the fact that first order structural errors will results only in a second order error in the energy (at least if the procedure outlined above is used).

Finally we want to point out that there is a way to correct the leading errors due to the Pulay stress. Evaluate the Pulay stress along the guidelines given above and add an input-line to the INCAR file which reads

From no on all STRESS output of VASP is corrected by simply subtracting PSTRESS. In addition, all volume relaxations will take PSTRESS into account (see sec. 7.22).