[Wannier] Omega minimum search divergence in wannier90

[Nicola Marzari]

Thanks Alex - Arash or Jonathan might comment better - here
are my observations

1) when the optimal parabolic step length is much larger than
the trial step length, it would be safer to choose a smaller step
(e.g. 2 or 3 times the trial) and also reset to steepest
descent the algorithm

2) it would be good to have a check that if the functional has
not gone down, U and M are restored to the previous step, we
go back to steepest descent, and the trial step length is halved
(so that even a large step now will be 2 or 3 times the half trial).

Alex, maybe you could even look into the code and fix these,

nic


Alex Kutana wrote:
> Dear wannier90 developers:
> 
> I have encountered the following problem during the search for the minimum of omega in wannier90: 
> at
> some step of the minimum search, the next omega is predicted to be
> smaller than the current one, whereas it turns out to be much larger, i.e. omega increases during the search.
> Since
> there seems to be no check on the actual (as opposed to predicted)
> value of the new omega, the whole search routine blows up from there.
> 
>  The increase
> in omega diagonal (O_D) occurs during step 3. Here are the values at the
> beginning of step 3:
> 
> LINE --> Spread at initial point       :   49.8723128210178
> LINE --> Spread at trial step          :   49.5779629623660
> LINE --> Slope along search direction  : -0.156333808720867
> LINE --> Trial step length             :   2.00000000000000
> LINE --> Optimal parabolic step length :   17.0690978644447
> LINE --> Spread at predicted minimum   :   48.5380742807289
> 
> However, at the end of the step:
> 
> O_D=     54.0158131 O_OD=      6.2368759 O_TOT=     77.7379108 <-- SPRD
> Delta: O_D=  0.2895663E+02 O_OD= -0.1091029E+01 O_TOT=  0.2786560E+02 <-- DLTA,
> 
> i.e. the diagonal component of omega tilda increased by 29.0!
> I
> suspect it may also be a compiler issue, since I couldn't get wannier90
> to run with gfortran on my machine, but it seems to run with no problem
> with ifort.
> Here is the link to the archive with my .win, .amn, .mmn, .eig, and .wout files:
> http://www.mediafire.com/?zx2cvkelcmx
> 
> Thank you.
> 
> Alex Kutana
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Prof Nicola Marzari   Department of Materials Science and Engineering
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