[Wien] Issue with different convergence results and QTL-B warnings

[Lyudmila Dobysheva]

09.12.2015 20:57, Michał Wojciechowski wrote:
> Currently I’m performing calculations for a chromium magnetic wheel  - Cr_8F_8(O_2CH)_16.
>J = (E_FM - E_AFM)/36.
> Approach 1.
> I start with PBE functional from a command line with:
> runsp_lapw -i 50 -ec 0.00001 -cc 0.001
> The calculations converge nicely with no warnings or errors.
> I then change the XC functional to B3LYP (option 47 in case.in0 and additional case.ineece file)
> and run the command:
> runsp_lapw -eece -i 50 -ec 0.00001 -cc 0.001
> The calculations converge nicely with no errors but there is a single warning:
> :WARN : QTL-B value eq.  11.12 in Band of energy  -2.28970  ATOM=   27  L=  0
> Atom 27 is carbon.
> Using -in1new I was able to determine, that using parameter -1.70 instead of 0.30 resolves the issue.
> I then changed entries for all carbon atoms
> and continued the calculations. This time I got no warnings and obtain results:
> E_FM = -24370.92259156 Ry
> E_AFM = -24370.92652933 Ry
> J = 1.5 meV
> Approach 2.
> I started with PBE functional and already modifie energy parameter in case.in1.
> After convergence is reached I continue with B3LYP functional. No warnings are present and
> I obtain the following results:
> E_FM = -24370.91735838 Ry
> E_AFM = -24370.92522898 Ry
> J = 3.0 meV

I am not an expert in B3LYP, so I am writing about only general 
considerations.
The reason may be
1) different execution. For example, you have made, by accident, for the 
final calculation in first case -in1new, in second - usual, without this 
option. Check the executions in dayfile's, :log's, and everywhere where 
this info can be found. And check all parameters. Compare the in-files 
in the whole saved directories.
Though system looks rather symmetric, there are 5 nonequivalent Cr 
atoms. Had you done force relaxation? Is struct files identical in both 
cases? in all four cases?
2) Maybe some parameter doesn't reach convergence, so small variations 
in calculation scheme (starting electron density) gives different 
results. Under suspicion, imho, RKM = 2.0 (as far as I remember, the 
recommendation for H-containing systems is RKM=3), very different RMT
2.40 and 0.83 Bohr for Cr and H, single k point.
3) maybe this exotic scheme of potential gives you really two different 
points? Look in case.scf other parameters, especially magnetic moments 
of Cr's, especially FM states (the difference is largest. For AFM it 
looks for me as not different).
3A) maybe this is a peculiarity of B3LYP scheme, and its iterative 
procedure allows to fix solution in some points that are not real 
minimum. Maybe users with experience with B3LYP can add something about 
this.
3B) Maybe this different solutions are really different minimums due to 
different Cr atoms with differently distributed moments among them.

Offtop:
 > J = (E_FM - E_AFM)/36.
Why 36? There is another number of atoms?

Best wishes
Lyudmila Dobysheva
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