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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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	max(ENMAX)	3/2×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.

• Plane wave kinetic energy cutoff (ENCUT):

The PREC-tag determines the energy cutoff ENCUT, if ENCUT is not specified in the Template:FIL 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). 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):

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
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

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