[Pw_forum] strange behaviour in LDA+U calculations

Giuseppe Mattioli giuseppe.mattioli at mlib.ism.cnr.it
Thu Jun 10 15:28:26 CEST 2010

Dear Matteo and Nicola
Thank you for your comments and suggestions
I'll try to work out something and, possibly, to report the results.

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

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