Dear Matteo and Nicola Thank you for your comments and suggestions I'll try to work out something and, possibly, to report the results. Giuseppe
On Wednesday 09 June 2010 21:32:35 Nicola Marzari wrote: > Dear Giuseppe, > > it looks like > > 1) PW does not converge PBE+U. Only things to try are really large > smearings, and > very small mixing factors. If the smearings are large, and the mixing is > small, it should ultimately > converge, and then you can reduce the smearing. For this purpose, > fermi-dirac is probably the best > choice, with its long tails occupying empty states. > > 2) glad that CP converged. the PBE+U solution, be it better or worse > than PBE, leads to > dissociation of the cluster - that's why damped dynamics leads to the > cluster fragmenting, and > canonical md shows it exploding. > > Nothing too unconventional - but the truth might be that these are very > difficult systems > to describe accurately in DFT - and PBE+U seems, for better or for > worse, lead to dissociation. > > What is the right solution ? Impossible to say. If the clusters are > stable experimentally, at one of > the magnetizations you tried, than a dissociated solution is certainly > wrong, and one that is not > is only probably wrong. > > Start from something really simple (maybe one Co at most), and compare > there PBE, PBE+U, > and quantum chemsitry (ideally MRCI) solutions. Does anything work ? > > These are incredibly difficult systems to study - have also a look at > e.g. Kulik et al JCP 2008. > > nicola > > Giuseppe Mattioli wrote: > > Dear all > > I'm facing strange problems while attempting to perform LDA+U > > calculations on Co-O clusters. PBE calculations work fine, both with PW > > and CP, both in closed shell and LSDA framework. In particular, > > ferromagnetic or antiferromagnetic initial guesses lead to closed shell > > electronic structures, i.e., total and absolute magnetization equal (or > > very close) to zero, and a safe insulating system. > > In order to improve the p-d hybridization between O 2p and Co 3d orbitals > > (their contributions to the DOS are quite separated in PBE calculations, > > and they are expected to be strongly mixed near to the valence band > > maximum) I've performed some PBE+U calculations by using reasonable > > values for the Co 3d and O 2p channels. In the case of closed shell > > calculations I obtain quite reasonable results, i.e., equilibrium > > geometries nicely similar to PBE ones, and an improved DOS in a better > > agreement with the available experimental data. I've tried to perform > > open shell calculations since January, I've tried to apply all of the > > standard and less standard tricks, but I have never (!) been able to > > converge a single calculation. There are no errors, they simply does not > > converge, with the system magnetization dancing around some selected > > values, depending on the initial guess (fm or afm). Notice that the > > system is still insulating (at a PBE+U closed shell level) and I've tried > > everything from fixed occupations to quite high smearing (gaussian, cold, > > ...) values. > > > > I thought that the CP code could help me, but I'm facing an even more > > strange problem... The very robust CP minimization of electronic dof > > successfully reach some ground state at the PBE+U level, the only (minor) > > problem being that you must put into the inpfile the final magnetization > > of the system. Then, I've tried to perform both damped and NVT (with Nose > > thermostats) MD simulations, trying several different open shell > > (nspin=2) configurations, ranging from tot_magnetization=0 to > > tot_magnetization=20. In the case of damped MD, the system temperature > > was kept very low, but my Co-O clusters tried to split into pieces (they > > are quite stable in both PBE and PBE+U PW calculations) while the total > > energy lowers. In the case of NVT MD thermostats went bananas and my > > clusters melt. You can scrub away Co and O scattered atoms from the > > supercell walls...:-) > > In the case of PBE calculations (both damped and NVT MD) everything went > > fine (no explosions, reasonably fast thermalization of the investigated > > systems, averaged bond distances similar to PW ones...) > > I can provide input files, but I suspect that this is not a matter of > > input failures... Is it LDA+U variational? I suspect it isn't... Is it > > foolish to perform MD calculation in such framework? Could you lend me a > > heavy hammer to persuade PW open shell calculations into convergence...? > > > > Thank you in advance for suggestions > > > > Giuseppe -- ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ?gaux en droits. Les distinctions sociales ne peuvent ?tre fond?es que sur l'utilit? commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la libert?, la propri?t?, la s?ret? et la r?sistance ? l'oppression. ******************************************************** ? ?Giuseppe Mattioli ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?CNR - ISTITUTO DI STRUTTURA DELLA MATERIA ? ? ?v. Salaria Km 29,300 - C.P. 10 ? ? ? ? ? ? ? ? ? ?I 00016 - Monterotondo Stazione (RM) ? ? ? ? ? ? ?Tel + 39 06 90672836 - Fax +39 06 90672316 ? ? ? ?E-mail: <giuseppe.mattioli at ism.cnr.it>
