[Wien] How to speed up a reliable optimization

Thu Jan 26 14:01:59 CET 2017

A clarification on some terms/types. For certain -eece and probably also +U
can stop at what should be called "traps". This is when :DIS and the plane
waves (:PLA) are self-consistent, but :MV is not. You can also have a trap
where :DIS is small but :PLA is not, and if you start from a converged
density but the forces are large. The latest mixer does a better job of
escaping these traps. I would not be surprised to find that there are
published results which are really traps. (Not just with Wien2k, with other
DFT codes as well.)

Local minima are "real" and "right". Wien2k (and most other codes) will
stop at local minima, only a few search for the global minimum. For certain
one can have local minima for spin states; it is not impossible to have
local minima for atomic positions. MSR1(a) can escape from small traps,
i.e. ones which are only present for a small range of densities/positions.
Sometimes small traps are due to noise in the algorithms.

Your InP case could find a local minimum, or a trap. Traps you can check by
looking to see if terms in addition to :DIS are converged.

mBJ is a special case that I don't understand -- I have not worked with it
enough to know if it has traps. I also don't know if the forces with mBJ
can be trusted. From some simple tests I did I suspect that they can't, but
I am not certain.

To escape a trap or a small local minimum you don't want to add SLOW,
decrease the GREED etc. -- use the larger default parameters.

To escape true local minima (spin or positions) you have to change to a new
starting point that is outside the radius of convergence (in
spin/position/density space) of your earlier calculation. There are some
emails on the list about changing spin states by hand.

---
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
http://www.numis.northwestern.edu
Corrosion in 4D http://MURI4D.numis.northwestern.edu
Partner of the CFW 100% gender equity project, www.cfw.org/100-percent
Co-Editor, Acta Cryst A

On Jan 26, 2017 04:18, "Luis Ogando" <lcodacal at gmail.com> wrote:

Dear Prof. Marks,

   Thank you very much for your answers ! I am pretty sure that your
"intuition" will save a lot of computation time !
   Just one last question: I have another system , a supercell formed by 15
InP zinc blend cells along [111] (hexagonal representation of the cubic
lattice) and 3 InP wurtzite cells. They are aligned along the hexagonal "c"
axis.
                                     Comparing the gaps of the respective
bulks and this supercell (same calculation parameters) , I believe that the
SCF cycle (here, it is not a lattice optimization) stopped at a local
minimum.
                                      My question is : in this case
(regular SCF cycle and InP cells), would you change any of your previous
answers ?
   Thank you again !!
   All the best,
                 Luis

2017-01-25 17:56 GMT-02:00 Laurence Marks <L-marks at northwestern.edu>:

> Inlined is my intuition, which does not have to be completely right.
>
> On Wed, Jan 25, 2017 at 11:32 AM, Luis Ogando <lcodacal at gmail.com> wrote:
>
>> Dear Prof. Marks (and Wien2k community),
>>
>>    After a recent discussion about "difficult" optimizations in this
>> mailing list (subject: "Mixer surprise when using PBE0 hybrid on-site
>> functional"), I would like ask you for an advice.
>>    I have a system with:
>> * 5 rings with C, H and N atoms
>> * 100 atoms with P1 symmetry
>> * the rings are out of a plane
>> * vacuum along y and z
>>    I know that this is a very hard optimization problem, so I would like
>> to kindly ask:
>>
>> 1) Do you believe that MSEC3a will work better than MSR1a in such a case ?
>>
>
> I would switch to MSEC3a, use SLOW or reduce the GREED to 0.1 only if you
> see indications of problems. If the system is a decent insulator and the
> experimental positions are quite good you may have no problems. At the end
> I would switch back to MSR1a certainly for a system with OH as the
> positions of the hydrogens can be quite soft. It can take quite some time
> to get the rotations of the OH bond distance right as Wien2k uses cartesian
> coordinates not polars. Which of the 3 is best -- I am not sure.
>
>>
>> 2) Do you recommend using -it, -vec2pratt and -noHinv options with
>> run_lapw ?
>>
>
> I use -it -noHinv -vec2pratt. Sometimes you need to do an occasional full
> diagonalization (touch .fulldiag) as the iterative method is less stable
> (it adds a little noise). In my personal version I have added back the old
> -itn option so this is done automatically every few steps.
>
>>
>> 3) Should I reduce TRUST to 0.5 (I am using LDA and experimental values
>> for the initial atomic positions) ?
>>
>
> Probably not. TRUST 0.5 would be if it is taking much too large steps
> which tends (in my experience) to occur more with soft electronic modes
> such as one has with d and f electrons.
>
>>
>> 4) Should I use SLOW in case.inm ?
>>
>
> See my answer to 1)
>
>>
>>    Many thanks in advance.
>>    All the best,
>>                       Luis
>>
>>
>
>
> --
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to think what nobody
> else has thought", Albert Szent-Gyorgi
> www.numis.northwestern.edu ; Corrosion in 4D:
> MURI4D.numis.northwestern.edu
> Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cfw.org_100-2Dpercent&d=CwMFaQ&c=yHlS04HhBraes5BQ9ueu5zKhE7rtNXt_d012z2PA6ws&r=U_T4PL6jwANfAy4rnxTj8IUxm818jnvqKFdqWLwmqg0&m=MNscdaGYE15HljeOPwtzDZH4S_JeFNBLtGtQWhVb-FY&s=kL1N7z6QnaXOzYazrQU6rH7eGamLhLbu0ncgZ2Hfz0E&e=>
> Co-Editor, Acta Cryst A
>
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