[Wien] How to speed up a reliable optimization
Laurence Marks
L-marks at northwestern.edu
Thu Jan 26 14:28:48 CET 2017
The last two (three) have useful information. They are
Mode Size Step
Mode is whether the trust region is active; it is off when Newton is shown.
Size is the radius of the trust region compared to a full Newton step, for
instance 0.5 would mean only half the full radius.
Step is the magnitude of the step taken compared to a full Pratt step.
All mixer codes (other DFT as well) use a Taylor series expansion for the
density change with some approximation for the Jacobian (see the mixer
README & papers). MSEC3 & MSR1 use different Jacobian approximations, as
did the old BROYD. Neglecting the higher-order terms is only valid for some
total step magnitude, the trust-region radius which is the "Size" variable
above. When Size is small the algorithm thinks these higher-order terms
matter. (The mixer adjusts the trust-region radius depending upon how much
the self-consistency is improving, which is very tricky, fuzzy coding and
imperfect.)
The numbers you sent indicate that the algorithm thinks everything is
"fine". If Size is small (e.g. 0.05) and remains small that suggests a
problem. The "Step" term can be small when :DIS is large, but should not be
small near the solution. If it is large (2-3 or more) the algorithm thinks
that there are soft modes present.
---
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 05:02, "Luis Ogando" <lcodacal at gmail.com> wrote:
Dear Prof. Marks,
Just for completeness, in the InP case I am using mBJ and I have checked
that during the last 20 regular SCF cycles the steps are changing from
:MIX : MSE1 REGULARIZATION: 6.30E-06 GREED: 0.317 Newton 1.00 0.22
to
:MIX : MSE1 REGULARIZATION: 4.93E-06 GREED: 0.437 Newton 1.00 1.14
Thank you once more !
All the best,
Luis
* PS: Could you, please, clarify the meaning of the last two columns in the
:MIX line ?
2017-01-26 8:18 GMT-02:00 Luis Ogando <lcodacal at gmail.com>:
> 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-percen
>> t
>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cfw.org_100-2Dpercent&d=CwMFaQ&c=yHlS04HhBraes5BQ9ueu5zKhE7rtNXt_d012z2PA6ws&r=U_T4PL6jwANfAy4rnxTj8IUxm818jnvqKFdqWLwmqg0&m=CZHTnM-OmVb7WVhK4QBSMqumiacPyU-LxrLt8-sw9bU&s=cyXhG2IW3LH9f7wuLmlZGAS47BrtGluAX3LnBa9-Mug&e=>
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