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Technical errors fall into four categories
In conclusion, to minimize errors one should use the same setting for
ENCUT, ENAUG, PREC, LREAL and ROPT throughout all calculations,
and these flags should be specified explicitly in the INCAR file.
In addition it is also preferable to use the same supercell for all
calculations whenever possible.
- Errors due to k-points sampling. This will be discussed in
section 8.6. Mind that the errors due to the k-points mesh
are not transferable i.e. a
k-points grid leads
to a completely different error for fcc, bcc and sc. It is
therefore absolutely essential to be very careful with
respect to the k-points sampling.
- Errors due to the cut-off ENCUT. This error is highly transferable,
i.e. the default cutoff ENCUT (read from the POTCAR file)
is in most cases safe, and one can expect that
energy differences will be accurate within a few meV
(see section 8.4). An exception is the stress tensor
which converges notoriously slow with respect to the size
of the plane wave basis set (see section 7.6).
- Wrap around errors (see section algo-wrap).
These errors are due to an insufficient
FFT mesh and they are not as well behaved as the errors due to the
energy cutoff (see section 8.4). But once again,
if one uses the default cutoff (read from the POTCAR file)
the wrap around errors are usually very small (a few meV per atom)
even if the FFT mesh is not sufficient. The reason is that
the default cutoffs in VASP are rather large, and therefore
the charge density and the potentials contain only small
components in the region where the wrap around error occurs.
- Errors due to the real space projection. Real space projection
always introduces additional (small) errors. These errors are
also quite well behaved i.e. if one uses the same real space
projection operators all the time, the errors are almost constants.
Anyway, one should try to avoid the evaluation of energy differences between
calculations with LREAL=.FALSE. and LREAL=On/.TRUE (see section
6.39). Mind that for LREAL=On (the recommended
setting) the real space operators
are optimized by VASP according to ENCUT and PREC and ROPT i.e. one gets
different real space projection operators if ENCUT or PREC is changed
(see section 6.39).
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