I have a question regarding the SYMPREC-parameter: I have found a case where changing it from the standard 1.e-05 to 1.e-04 changes the energy quite significantly, from
E0= -.79707341E+03 (correct value, as far as I can tell) to
E0= -.79920458E+03
Looking at the OUTCAR (which I cannot upload because it is too large), the only differences I see in the setup (before iteration begins) are:
< SYMPREC = 1.00e-04
> SYMPREC = 1.00e-05
512c512
< Subroutine GETGRP returns: Found 16 space group operations
> Subroutine GETGRP returns: Found 8 space group operations
518d517
< The point group associated with its full space group is D_4h.
542c541
< Subroutine GETGRP returns: Found 16 space group operations
> Subroutine GETGRP returns: Found 8 space group operations
548d546
< The point group associated with its full space group is D_4h.
572c570
< Subroutine GETGRP returns: Found 16 space group operations
> Subroutine GETGRP returns: Found 8 space group operations
578d575
< The point group associated with its full space group is D_4h.
581c578
< Subroutine INISYM returns: Found 16 space group operations
> Subroutine INISYM returns: Found 8 space group operations
When the simulation starts (OSZICARs attached), energies are the same, but then they drift apart significantly.
I do understand that choosing a value of SYMPREC that is too large may force an incorrect symmetry on the charge distribution, but I am puzzled by the effect being so large here and by the fact that the energy with too large SYMPREC value is lower (shouldn't it be higher because there are less DOFs to minimize when we enforce a symmetry?).
I find it also a bit worrying considering that "change SYMPREC" is advice frequently given when a VASP run has trouble with the symmetry.
I've attached the INCAR and POSCAR and the 2 OSZICAR results; I have not attached the POTCAR because I think that's forbidden, the potentials used were
TITEL = PAW_PBE Ni_pv 06Sep2000
TITEL = PAW_PBE Nb_sv 25May2007
