Changing the PREC parameter influences the default for three different input parameters, and it is also possible to obtain the same results by changing the corresponding parameters in the INCAR file (VASP.4.X) and/or param.inc file (VASP.3.X) directly.
Mind: In vasp.4.4 the behaviour for PREC=High was changed: For PREC=High, ENCUT is set to the maximal ENMAX value in the POTCAR file plus 30%. This guarantees that the absolute energies are converged to a few meV, and it also enshures that the stress tensor is converged within a few kBar. If you want to get completely ``water-proof'' results PREC=High is recommended, but computational cost can be rather larger. In general PREC=High is never required for relaxations, and if the cell shape is not allowed to change. But the calculations of elastic properties should often be done in the PREC=High mode. If PREC=High turns out to be too expensive, one can also try to increase only the energy cutoff manually by 30 %, and stick to PREC=Med. In all tests we have done this was sufficient to obtain accurate results. An economical way to use PREC=High is to relax the structure first with PREC=Med and to reevaluate then the final energy with PREC=High.
This tag is also important for the automatic generation of the param.inc file using the utility
> makeparam(see section 6). For PREC=High wrap around errors are avoided (see section 9.2). For PREC= Low or Medium the 3/4 rule is used (see section 10.4), i.e. NGX,Y,Z is set to 3/4 of the value, which avoids wrap around errors. The 3/4 rule results in small (usually less than 1 meV/atom) not easily transferable errors.
The same holds for vasp.4.X, i.e. for PREC=High wrap around errors are avoided, (see section 9.2) and for PREC= Low or Medium NGX,Y,Z is set to 3/4 of the value, which avoids wrap around errors. NGX,Y,Z can also be set manually in the INCAR file in oder to overwrite the defaults (see section 7.2).
|PREC= Low||700 points in the real space sphere (ROPT=0.67)|
|PREC= Med||1000 points in the real space sphere (ROPT=1.0)|
|PREC= Low||1500 points in the real space sphere (ROPT=1.5)|