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During a geometrical optimization of ions and lattice parameters in VASP, the forces acting on atoms were minimized to less than 0.01 eV/Å. However, when performing the subsequent self-consistent field (SCF) calculation using the optimized geometry 'CONTCAR', the forces printed in the OUTCAR file were found to be larger than the specified force convergence criteria used during the relaxation process.

The main inputs in the INCAR file for geometrical relaxation and SCF step are almost the same. Is that a normal behavior? What are the possible reasons for this discrepancy?

The following is the INCAR file for Geometrical Relaxtion:

ISTART = 0
ICHARG = 2
NELM   = 100
EDIFF  = 1E-6
ENCUT  = 400
ALGO   = Fast
IBRION = 2
ISIF   = 4
POTIM  = 0.5
NSW    = 300
EDIFFG = -0.01
ISMEAR = 0
SIGMA  = 0.05
PREC   = Accurate
LREAL  = Auto
LWAVE  = .F.
LCHARG = .F.
GGA    = PE
IVDW   = 11
ISYM   = 0

while the INCAR file for the SCF calculation is the folloing:

ISTART = 0
ICHARG = 2
NELM = 100
EDIFF = 1E-6
ENCUT = 400
ALGO = Fast
IBRION = -1
ISIF = 2
POTIM = 0.5
NSW = 0
EDIFFG = -0.01
ISMEAR = -5
SIGMA = 0.2
PREC = Normal
ISPIN = 2
LREAL = .F.
LWAVE = .T.
LCHARG = .T.
LAECHG = .TRUE.
LDIPOL = .T.
IDIPOL = 3
GGA  = PE
IVDW = 11
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  • $\begingroup$ Can you include your input files for the geometry optimization and the SCF? Also, is the latter calculation just an SCF or are you actually requesting that forces be calculated? $\endgroup$
    – Tyberius
    Nov 12, 2023 at 3:06
  • $\begingroup$ @Tyberius I have added the samples of the INCAR files for both geomtrical relaxation and SCF calculation. $\endgroup$ Nov 12, 2023 at 6:55
  • $\begingroup$ I don't know much about VASP settings, but the difference in forces might be related to the PREC setting. From the vaspwiki: "PREC=Accurate increases the memory requirements somewhat, but it should be used (in combination with an increased value for ENCUT) when a very good accuracy is required, e.g., for accurate forces..." $\endgroup$
    – Tyberius
    Nov 12, 2023 at 16:36
  • $\begingroup$ @Tyberius I have repeated the calculation with PREC=Accurate and changed back to Gaussian smearing, I have also removed the dipole correction, with these changes the SCF inputs are exactly the same except for that I am running a single-point calculation. Regardless of that the lowest value I got for the force was 0.012306 which is still not less than 0.01. $\endgroup$ Nov 12, 2023 at 23:54
  • $\begingroup$ @TyberiusI know that the forces acting on the atoms are calculated by taking the derivative of the total energy with respect to the atomic coordinates. However, as the atoms are fixed in a single-point calculation, do we still need to care about the Forces printed in the OUTCAR file at this stage? that's mainly what my question is. $\endgroup$ Nov 12, 2023 at 23:54

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