Hi all, This is a copy of an e-mail exchange between Carlo and I that addresses some of the questions in this thread. Sorry I missed it the first time!
> I'm trying to do periodic PW calculations on solid structures using Quantum > Espresso 5.1.1 with pbesol - PAW pseudopotentials on some systems that can > contain C H N I Br Cl F atoms characterized by the presence of halogen > bonds (N ... I). > Since it is reported that these parameters "[fix] and [old]", and "Can be > re-parametrized on demand", I'm wondering whether this is necessary (should > I use different a1/a2 parameters?), and/or you can give me an advice. > The xdm + pbesol seems to overestimate the attractive forces, while > pbesol underestimate them. PBEsol is not a particularly good functional when combined with dispersion corrections (or at least with XDM). I suspect the reason is that PBEsol was designed with the purpose of nailing the equilibrium cell parameters by sacrificing accuracy in other departments (like, for instance, cohesive energies). This results in a functional that is more binding than PBE, which is not good news when you consider non-covalent interactions. Particularly, donor-acceptor interactions like halogen bonds. You can get an idea of the accuracy of PBEsol by checking the MAPD in the parametrization fit on gatsby (18.9%). The "parametrized on demand" bit on the webpage is not very well phrased. The parameters marked with the [old] tag were obtained with a previous implementation of XDM in QE, in which the atomic volumes were calculated in a different way. When I wrote the XDM bit that is currently in the QE distribution, I reparametrized PW86PBE, B86BPBE, PBE, and BLYP, but didn't bother with the other two because I figured that they would not be very useful. The [fix] tag means that I had to force the a1 to be zero because the a1/a2 parametrization would find a minimum MAPD at negative a1, which is unphysical. This is a clear sign that things are not going well with the base functional. If you need those a1/a2, I could easily run the parametrization calcs for you and find them, although I don't expect things will improve much with the new parameters. My advice is, if possible, to use one of the XDM-corrected functionals that work better for non-covalent interactions, such as PW86PBE (11.7% MAPD) or B86BPBE (11.8%). > My first structure optimized with a smaller volume (1685 a.u.^3) than the > real one (1835 a.u.^3) (if you wish I can send the input file for pw.x). > I've been used the a1 and a2 parameters shown in the web pages for pbesol > (a1=0.0, a2= 4.1503). In any case, halogen bonds are a tough cookie in a periodic solid. This is because all dispersion-corrected GGAs overbind halogen bonds--both in terms of intermolecular distance and interaction strength. The reason is delocalization error, as explained here: http://pubs.acs.org/doi/abs/10.1021/ct500899h The article shows that all GGAs present delocalization error by about the same amount, and that halogen bonds can only be correctly modeled (or rather, modeled at the same accuracy level as other non-covalent interactions) if you use a dispersion-corrected half-and-half functional. This is naturally a difficult thing to do in QE. So if N...I non-covalent contacts are part of your crystal framework, I'm not surprised at all that PBEsol-XDM grossly underestimates the volume. I would suggest to go with one of the functionals that work better for NCIs and see how that goes. > Besides the structures, I 'm interested in phonons and Solid State NMR > (since our group is doing SSNMR). I know that I would need to take in > considerations the relativistic effects (i.e. ZORA), but at the moment this > kind of calculation in not available, The reason I didn't implement the nspin > 2 case is that I'm not sure how to do the bit about the kinetic energy density, and I didn't have much use for that bit at the time. If this problem can be circumvented, then the implementation should be straightforward, but I have no idea how to go about it off the top of my head. Best, A. --- Dr. Alberto Otero de la Roza National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
