[Pw_forum] strange behaviour in LDA+U calculations

[Giuseppe Mattioli]

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                            
   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>