PREC
PREC = Low | Medium | High | Normal | Single | Accurate
| Default: PREC | = Medium | for VASP.4.X |
| = Normal | for VASP.5.X |
Description: PREC specifies the "precision"-mode.
We recommend to use PREC=Normal for calculations using VASP.4.5 and higher (the default in VASP.5.X) and PREC=Medium for VASP.4.4. PREC=Accurate avoids wrap around errors and uses an augmentation grid (NGXF×NGYF×NGZF) that is exactly twice as large as the coarser grid (NGX×NGY×NGZ) used 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.
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 increased energy cutoff.
Mind: we recommend to specify the energy cutoff (ENCUT) always manually in the INCAR file, to avoid incompatibilities between calculations.
The PREC parameter sets the defaults for four sets of parameters (ENCUT; NGX, NGY, NGZ; NGXF, NGYF, NGZF, and ROPT):
where max(ENMAX) and max(ENMIN) are the maximum ENMAX and ENMIN found on the POTCAR file, and
with Ecut=ENCUT and Eaug=ENAUG.
Mind: the settings PREC=Normal and PREC=Accurate are only available in VASP.4.5 and newer versions. The setting PREC=Single is only available as of VASP.5.1.
- Plane wave kinetic energy cutoff (ENCUT):
- The PREC-tag determines the energy cutoff ENCUT, if ENCUT is not specified in the INCAR file. For PREC=Low, ENCUT will be set to the maximal ENMIN value found on the POTCAR file. For PREC=Medium and PREC=Accurate, ENCUT will be set to maximal ENMAX value found on the POTCAR file. Finally for PREC=High, ENCUT is set to the maximal ENMAX value on the POTCAR file plus 30%. PREC=High guarantees that the absolute energies are converged to a few meV, and it ensures that the stress tensor is converged within a few kBar. In general, an increased energy cutoff is only required for accurate evaluation of quantities related to the stress tensor (e.g. elastic properties).
- For PREC=High and PREC=Accurate, wrap around errors are avoided, all G-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 (NGXF, NGYF, NGZF). 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 files (for scanning tunneling microscopy simulation, it is recommended to use PREC=Single). In all other cases, NGXF, NGYF, and NGZF are determined by some rather heuristic formula from ENAUG.
- Real space projectors (ROPT):
Related Tags and Sections
NGX, NGY, NGZ, NGXF, NGYF, NGZF, ENCUT, ENAUG, ENMAX. PRECFOCK