PREC= Low | Medium | High | Normal | Accurate | Single
The settings Normal and Accurate are only available in VASP.4.5 and newer versions. The setting Single is only available in VASP.5.1.
Changing the PREC parameter influences the default for four sets of parameters (ENCUT; NGX, NGY, NGZ; NGXF, NGYF, NGZF and ROPT), and it is also possible to obtain the same characteristics by changing the corresponding parameters in the INCAR file (VASP.4.X) directly.
The following table summarizes how PREC determines other flags in the INCAR file:
For PREC=High and PREC=Accurate, wrap around errors are avoided (see section 7.2, all -vectors that are twice as large as the vectors included in the basis set are taken into account in the FFT's). For PREC=Low, PREC=Medium or PREC=Normal, the FFT grids are reduced, and 3/4 of the required values are used. Usually PREC=Medium and PREC=Normal, are sufficiently accurate with errors less than 1 meV/atom.
In addition, the PREC tag determines the spacing for the grids representing the augmentation charges, charge densities and potentials (NGFX, NGFY, NGFZ). For PREC=Accurate and PREC=Normal, the support grid contains twice as many points in each direction as the grids for the orbitals (NGXF= 2 NGX, NGYF= 2 NGY, NGZF= 2 NGZ). PREC=Single is identical to PREC=Normal, execpt that the double grid technique is not applied. This is convenient of you need to cut down on storage demands, or want to reduce the size of the CHG and CHGCAR file (for scanning tunneling microscopy simulation, it is recommended to use PREC=Single). In all other cases, they are determined by some rather heuristic formula from ENAUG (see Sec. 6.10).
for LREAL=O the defaults are:
|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= Normal||1000 points in the real space sphere (ROPT = 1.0)|
|PREC= Accurate||1000 points in the real space sphere (ROPT = 1.0)|
|PREC= High||1500 points in the real space sphere (ROPT = 1.5)|
This behaviour can be overwritten by specifying the option ROPT in the INCAR file. For mixed atomic species we, in fact, strongly recommend to use LREAL=A (see section 6.39).
PREC=Accurate avoids wrap around errors and uses an augmentation grid that is exactly twice as large as the coarse grid for the representation of the pseudo wavefunctions. PREC=Accurate increases the memory requirements somewhat, but it should be used if very accurate forces (phonons and second derivatives) are required. The accuracy of forces can be further improved by specifying ADDGRID = .TRUE. (see Sec. 6.63).
New manual entry for PREC=High:
The use of PREC=High is no longer recommend (and exists only for compatibility reasons). For an accurate stress tensor the energy cutoff should be increased manually, and if additionally very accurate forces are required, PREC=Accurate can be used in combination with an increase energy cutoff. Note, that we now recommend to specify the energy cutoff always manually in the INCAR file, to avoid incompatibilities between calculations (see Sec. 6.2.3).
Old manual entry for PREC=High:
PREC=High, should be used if properties like the stress tensor are evaluated. If PREC=High calculations are too expensive, ENMAX can also be increased manually in the INCAR file, since this is usually sufficient to obtain a reliable stress-tensor.