Hello Florian, I tried to perform similar calculations some time ago, and found the same problems. I am not 100% sure, but I think that with ph.x forces are computed without accounting for the dispersion interaction. This would mean that you are basically calculating phonons with a crystal structure that is not at the minimum energy relative to the xc potential you are using (since the relaxation is performed WITH vdW).
I hope that someone says that I am wrong! Kind regards, Carlo -- Carlo Motta PostDoc Researcher School of Physics and CRANN, IE Mob.: +353 (0) 83440 8930 Trinity College Dublin, IT Mob: +39 3286845055 Dublin 2, IRELAND http://www.spincomp.eu/members/carlo-motta.xml ________________________________________ Da: [email protected] [[email protected]] per conto di Florian Altvater [[email protected]] Inviato: mercoledì 5 novembre 2014 11.51 A: [email protected] >> PWSCF Forum Oggetto: [Pw_forum] Phonons with DFPT and DF2 functional Hi, I am interested in looking at effects of using van der Waals functionals like vdW-DF2 on the phonon dispersion of organic crystals. Preliminary results indicate that there might be some issues with using DFPT and DF2 together. More specifically, DFPT gives a couple "very imaginary" frequencies (below -100cm^-1) for Gamma-point phonons (with and without acoustic sum rule), while finite displacement results seem to be just fine, using the same convergence parameters. So before I dig deeper into eventual other issues at play here, I wanted to ask if you know of any incompatibilities when using DFPT and vdW-DF2? Either inherently embedded in the underlying theory (which I could then dig into first), or just in the way it is implemented in QE. Thanks for your help, Florian --- Florian Altvater PhD candidate UC Berkeley/LBNL _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
