[Pw_forum] strange behaviour in LDA+U calculations

[Nicola Marzari]

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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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu