Dear PWSCF community, I already asked these questions a good while ago, but unfortunately none of the resposes solved my problem. This would be the following:
I want to study relative 'simple' proton transfer reaction on a ZnO surface (surface slab represented with a hexagonal supercell and ultrasoft psp). When i do an NEB calculation, the wfc for those images, which are close either to the reactant, or the product converge really nicely within 7-12 SCF steps. Such an example, if i make a band with 17 images (15 movable 2 fixed), than the wfc for images 2->6 and 9->15 converges flawlessly and really fast. Now, for images 7-8-9 i cannot manage to get the wfc to converge, no matter what i do. Of course, one could argue, that the position of the nuclei in these configuration are way too far from equilibrium. I completely agree with this, but if i visualize them, they are completely meaningful TS configurations, where an OH group is elongated to around 1.6 A and this proton is oriented towards a 'target' C or O atom, which would give me the product. What is even more puzzling, is that after the first linear interpolation (when the nuclear positions are the worse possible) the wfc does converge for all images most of the time. After 4-10 iterations with the band the barrier nicely goes down to around 2-3 eV, because the path is getting better and better. This is exactly the point, when this strange behavior in the wfc convergence kicks in. For images 7-8-9 the Total Energy and the gradient on the wfc will just fluctuate up and down within a given range, without achieving convergence ever. What i tried up to now: decrease mixing_beta to values as low as 0.1, tried all the mixing modes (my default was mixing mode=?local-TF?), increased mixing ndim to 12, increased ecutrho to 5 times ecutwfc. Altough, the ZnO slab should be semiconductor, just to try to get a converged wfc i added around extra 20% empty bands and a broadening. This did not help, either. Because of the available memory i do not really wanna play around to crank up the cutoff, but if one say that this gonna help, i can give it a try. Just for testing, i plugged the same geometries with the same VDB psp with the same cutoff values into CPMD. Although the achieving self-consistency was slow as well, but still i could obtain a converged wfc and calculate the forces for these configurations without too much additional effort/trick. My point here is not that CPMD is better/worse, but the fact that the given configurations were not completely nonsense, and the whole setup and set of psp is reasonable. Is this behavior completely normal in an NEB calculation with PWSCF, or this is an issue with my particular system/setup only? What else could one try, to get the wfc somehow converged? Another question: when i restart an NEB calculation, how can i restart the wfc for those images, which were already converged for a given NEB iteration? The output file indicates, that the initial density and the Starting wfc were read in 'from file'. I would have expected, that once this is done, the wfc will converge within 1-2 iterations, because the already self-consistent wfc was written out in the respective *.wfc files. Apparently this is not the case, because even for those images, where the wfc was converged, i still nead to spend again like 5-9 SCF cycles/image. What am i doing wrong? Yours Sincerely, Janos. ================================================================== Janos Kiss e-mail: janos.kiss at theochem.ruhr-uni-bochum.de Lehrstuhl fuer Theoretische Chemie Phone: +49 (0)234/32-26485 NC 03/297 +49 (0)234 32 26754 Ruhr-Universitaet Bochum Fax: +49 (0)234/32-14045 D-44780 Bochum http://www.theochem.ruhr-uni-bochum.de ==================================================================