Compared to say the DFT-D3 type of dispersion corrections (set with IVDW in VASP), I've found the nonlocal vdW-DF functionals (described here in the VASP Wiki) to be much more difficult to converge to similar levels of accuracy (which I guess should be expected given the nature of the vdW correction). My monolayer calculations with the optB88-vdW functional led to some geometry optimizations with several consecutive unconverged electronic optimization steps (even with NELM = 90, i.e. 90 SCF steps), when starting from a converged structure from a database.

My first impression was that my use of EDIFF=1E-07 (VASP's electronic convergence parameter) and EDIFFG = -0.002 (VASP's geometry convergence parameter) was too fine, which seemed somewhat in line with what I thought was a rougher convergence criterion by 2DMatPedia (a 2D materials database which used the optB88-vdW functional and an ionic convergence criterion that the energy difference is below $10^{-4}$ eV for structure relaxations). However, the JARVIS database, which also contains 2D monolayers, seemed to be able to do calculations with optB88-vdW and (what I thought were) stricter convergence criteria:

"Both the internal atomic positions and the lattice constants are allowed to relax in spin-unrestricted calculations until the maximal residual Hellmann–Feynman forces on atoms are smaller than $0.001$ eV Å$^{−1}$ and energy-tolerance of $10^{-7}$ eV with accurate precision setting (PREC=Accurate)."

This leads me to my question.

Are the convergence difficulties I've faced with optB88-vdW expected (and so it is reasonable to relax my convergence criteria)? Or are there other things that I can tweak to achieve the tighter convergence criteria that would be important for later phonon and elastic constants calculations for example?

I have played around with just trying to converge static calculations tightly (EDIFF=1E-07), by changing the electronic minimization algorithm to ALGO = Normal or ALGO = ALL (I usually use ALGO = Fast), or playing around with IMIX (density mixing). However, I could not find a convenient general setting for successful convergence all the time. Different combinations have helped me achieved convergence for the 4-6 different monolayers I've tested, but I had to 'babysit' each calculation.

  • $\begingroup$ Most of the qualitative results are fine with EDIFF= 10-5 and EDIFFG=-0.01 eV/Ang. EDIFF=10-4 is little relaxed but fine in the extreme case $\endgroup$ Commented Nov 2, 2022 at 11:19
  • $\begingroup$ @Pranavkumar Thanks for your input. I'd agree with you for energy-based results (defect formation energy, exfoliation energy, etc), but I would think that tighter convergence is necessary for quantities like force constants and elastic constants that depend on how forces and stresses change with small perturbations. My impression is that the forces and stresses have separate convergence behavior that depends more on the convergence of the orbitals (which are tighter than the usual convergence criteria based on the energies), e.g. VASP Wiki mentions EDIFF = 1E-7 being required for phonons. $\endgroup$
    – CW Tan
    Commented Nov 2, 2022 at 15:36

1 Answer 1


I've managed to converge my optB88-vdW geometry relaxations to within forces of 0.002 eV/Å (EDIFFG=0.002), with an electronic convergence tolerance of $10^{-6}$ eV (EDIFF=1E-6). This was achieved partly by using a finer $\mathbf{k}$-point mesh and the conjugate gradient algorithm (IBRION=2), with ALGO=Fast. I think my convergence difficulties stemmed from using the qusi-Newton algorithm (IBRION=1) with the default step-size (POTIM=0.5) which should have been tuned.

For static calculations, I found that ALGO=All has been successful most if not all of the time.

My subsequent phonon and elastic constants calculations seemed to work out fine with my "tighter" geometry relaxations. However, I did find this paper titled "Energy, Phonon, and Dynamic Stability Criteria of Two-Dimensional Materials" (ACS Appl. Mater. Interfaces 2019, 11, 28, 24876–24884) which used optB88-vdW in VASP with ionic relaxation force tolerances of 0.01 eV/Å. They managed to get nice-looking phonon dispersion curves, which perhaps indicates that a higher force tolerance may be good enough.


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