Hi Stephan, In short, it is completely up to you to decide when convergence is reached.
In long, yes, it is quite an arbitrary choice. However, you should let literature guide you what are usual convergence thresholds for the parameter of interest. Depending on the code, plane waves, numerical orbitals, etc, you can achieve better or worse accuracy (e.g., meV, micro-eV). So, how to proceed? Do your calculations for different mesh-cutoffs, just as you did before. Do one extremely converged calculation, i.e. a very large value where you don't see any "sensible" change anymore in the numerical values. This concept of "sensible" is related to the code-dependent accuracy that I mentioned above. Decide on your convergence criterion, absolute or relative, in comparison with this one calculation. These criteria can for instance be "0.1% error compared to that one calculation" or "within 10 meV error compared to that one calculation". Then, find the value of meshcutoff that fits this criterion (while making sure of course that larger cutoff values also fit the criterion). And mention this convergence criterion in your publication! Note that convergence studies should be done for the parameter of interest for your study. It's not because convergence is reached for the total energy that it will be the case for your bond-length, or your band-gap, or your phonon spectrum, etc... As a rule of thumb, energy usually converges much faster than the rest. Good luck, Nicolas On Wed, Aug 12, 2015 at 7:12 PM, Mayuri Vaghela <[email protected]> wrote: > when you take any convergence if you draw mesh-cutoff vs energy graph , > you will get minima in the graph put that value of meshcutoff as your > converged mesh cut off value > > On Tue, Aug 11, 2015 at 10:21 PM, <[email protected]> wrote: > >> Hi Stephan, >> it depends on how big is your unit cell and what you want to study. >> >> > Hi, >> > >> > I've a question concerning the convergence criterion. >> > >> > Let's say I want to examine convergence of the total energy a system >> with >> > respect to the meshcutoff. >> > >> > When is convergence reached? Is it reached when there is no change in >> the >> > first digit after the comma >> > >> > like in the following example: >> > >> > meshcutoff total energy >> > >> > 150 -42994.1019 >> > >> > 170 -42994.116 >> > >> > 190 -42994.127 >> > >> > 210 -42994.133 >> > >> > >> > >> > or is it usefull to continue the test till there is no change in the >> > second digit after the comma? >> > >> > What is a sensible criterion for convergence? >> > >> > >> > >> > Thanks and regards >> > >> > >> > >> > Stephan Ludwig >> > >> > >> >> >> ********************************************************* >> Dr Zacharias G. Fthenakis >> Research Associate >> Institute of Electronic Structure and Laser (I.E.S.L.) >> Foundation for Research and Technology Hellas (FO.R.T.H.) >> Vassilika Vouton P.O. Box 1527 71003 Heraklion Crete Greece >> Phone +30 2810 391824 >> FAX +30 2810 391305 >> webpage: http://esperia.iesl.forth.gr/~fthenak >> ********************************************************** >> >> >
