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.

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We recommend to use PREC=Normal or PREC=Accurate. The tags PREC=Medium and PREC=High (available already in VASP.4.4) are no longer recommended. PREC=Normal can be used for most routine calculations, whereas PREC=Accurate reduces egg-box effects and errors even further. Specifically, PREC=Accurate strictly avoids any aliasing or 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 orbitals. 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 sometimes further improved by specifying ADDGRID=.TRUE. (but user reports are somewhat contradictory).

The use of PREC=High is no longer recommend (and is only available for compatibility). Essentially PREC=High only increases the energy cutoff by 30 %. The same effect can be achieved by just manually increasing ENMAX in the INCAR file. 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.

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The PREC parameter sets the defaults for four sets of parameters (ENCUT; NGX, NGY, NGZ; NGXF, NGYF, NGZF, and ROPT):

PREC	ENCUT	NGX,Y,Z	NGXF,YF,ZF	ROPT
Normal	max(ENMAX)	3/2×Gcut	2×NGX	-5×10-4
Single (VASP.5)	max(ENMAX)	3/2×Gcut	NGX	-5×10-4
Single (VASP.6)	max(ENMAX)	2 ×Gcut	NGX	-5×10-4
Accurate	max(ENMAX)	2×Gcut	2×NGX	-2.5×10-4
Low	max(ENMIN)	3/2×Gcut	3×Gaug	-1×10-2
Medium	max(ENMAX)	3/2×Gcut	4×Gaug	-2×10-3
High	1.3×max(ENMAX)	2×Gcut	16/3×Gaug	-4×10-4

where max(ENMAX) and max(ENMIN) are the maximum ENMAX and ENMIN found on the POTCAR file, and

$E_{\rm cut}=\frac{\hbar^2}{2m_e}G_{\rm cut}^2 \qquad E_{\rm aug}=\frac{\hbar^2}{2m_e}G_{\rm aug}^2$

with E_cut=ENCUT and E_aug=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, and has been updated in VASP.6.

• 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).

• FFT grids (NGX,NGY,NGZ,NGXF,NGYF,NGZF):

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). In all other cases, NGXF, NGYF, and NGZF are determined by some heuristic formula from ENAUG (these settings should be avoided).

PREC=Single is uses the same FFT grids as PREC=Normal in VASP.5 and the same FFT grids as PREC=Accurate in VASP.6. The double grid technique is not used. This is convenient, if one needs to cut down on storage demands, or if one wants to reduce the size of the files CHG and CHGCAR. Furthermore, PREC=Single avoids high frequency oscillations caused the double grid technique and the resulting Fourier interpolation. It is often expedient for scanning tunneling simulations or other calculations where high frequency wiggles of the charge density in the vacuum region are undesirable.

• Real space projectors (ROPT):

LREAL=O			LREAL=A
PREC	ROPT		PREC	ROPT
Low	0.67		Low	-1×10-2
Med	1.0		Med	-2×10-3
Normal	1.0		Normal	-5×10-4
Single	1.0		Normal	-5×10-4
Accurate	1.0		Accurate	-2.5×10-4
High	1.5		High	-4×10-4

Related Tags and Sections

NGX, NGY, NGZ, NGXF, NGYF, NGZF, ENCUT, ENAUG, ENMAX, PRECFOCK

Examples that use this tag

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