PREC-tag

PREC = Low | Medium | High | Normal | Accurate | Single

Default:	Medium for VASP.4.X
	Normal for VASP.5.X

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 PREC-flag determines the energy cutoff ENCUT, if (and only if) no value is given for ENCUT in the INCAR file. For PREC=Low, ENCUT will be set to the maximal ENMIN value found in the POTCAR files. For PREC=Medium and PREC=Accurate, ENCUT will be set to maximal ENMAX value found on the POTCAR file (see 5.4). Finally for PREC=High, ENCUT is set to the maximal ENMAX value in 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).

  The following table summarizes how PREC determines other flags in the INCAR file:

  PREC	ENCUT	NGx	NGx F	ROPT
  Normal	max( ENMAX)	3/2 $G_{\rm cut}$	2 NGx	-5E-4
  Single	max( ENMAX)	3/2 $G_{\rm cut}$	NGx	-5E-4
  Accurate	max( ENMAX)	2 $G_{\rm cut}$	2 NGx	-2.5E-4
  Low	max( ENMIN)	3/2 $G_{\rm cut}$	3 $G_{\rm aug}$	-1E-2
  Med	max( ENMAX)	3/2 $G_{\rm cut}$	4 $G_{\rm aug}$	-2E-3
  High	max( ENMAX)*1.3	2 $G_{\rm cut}$	16/3 $G_{\rm aug}$	-4E-4

  
  

  $$\frac{ \hbar^2}{2 m_e} \vert G_{\rm cut}\vert^2 = {\tt ENCU... ... \frac{ \hbar^2}{2 m_e} \vert G_{\rm aug}\vert^2 = {\tt ENAUG}$$
  
  
  max( ENMAX/ ENMIN) corresponds to the maximum ENMAX/ ENMIN found in POTCAR
  ENAUG defaults to the maximum EAUG found in POTCAR

• FFT-grids ( NGX, NGY, NGZ and NGFX, NGFY, NGFZ):

  For PREC=High and PREC=Accurate, wrap around errors are avoided (see section 7.2, all $\vec G$-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 (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 wavefunctions ( NGXF= 2 $\times$ NGX, NGYF= 2 $\times$ NGY, NGZF= 2 $\times$ 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. 6.9).

• If real space projectors are used, ROPT (which controlls the number of grid points within the integration sphere around each ion, see Sec. 6.38) is set to

  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)

  
  
  
  For LREAL=A the defaults are:

  PREC= Low	ROPT=-1E-2
  PREC= Med	ROPT=-2E-3
  PREC= Normal	ROPT=-5E-4
  PREC= Accurate	ROPT=-2.5E-4
  PREC= High	ROPT=-4E-4

  
  
  

  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 6.38).

We recommend to use PREC=Normal for calculations in VASP.4.5 (default in VASP.5.X) and PREC=Medium for VASP.4.4.

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. 6.56).

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.