Dear all, I've been trying to calculate band gaps for some semiconductor oxides using hybrid functionals (pbe0, hse, b3lyp). I've learned the choice of the q-grid for computing the exchange energy can be critical. Indeed in some cases I cannot achieve self-consistency and an error message appears in the output file warning that the dexx correction term becomes negative. I guess this is related to the small-q divergence issue for some particular q-grid patterns. In fact, if the k-grid is shifted away from gamma (keeping the q-grid fixed) convergence is achieved. Unfortunately, this doesn't allow me to compute the gap at the gamma point, which I'm interested in. On the other hand, I also obtain convergence if I set "x_gamma_extrapolation=.false.", but this induces large errors for a small number of points in the q-grid. How is it possible to solve the problem and, more importantly, predict in which configurations of the q-grid could the problem arise? Is there any way to shift the q-grid away from gamma?
Thanks for any help you can provide. Best regards, Matteo Gerosa -- Matteo Gerosa NanoLab - Micro and Nanostructured Materials Lab Department of Energy, Politecnico di Milano Via Ponzio 34/3 I-20133 Milano, Italy tel. +39 02 2399 6364 web: www.nanolab.polimi.it
