[Pw_forum] strange behaviour in LDA+U calculations

[Giuseppe Mattioli]

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



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   Giuseppe Mattioli                            
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