[Wien] Fixing Atomic Positions During Geometry minimization:constraint failed!
Laurence Marks
laurence.marks at gmail.com
Wed May 7 11:10:51 CEST 2025
Addendum: why fixing atoms does not do what you might think.
Some people think that if they fix some atoms then the calculations will
converge faster. This is incorrect, and is based upon a misunderstanding of
minimization.
In general the speed of minimization does not depend upon the number of
variable atom positions. For just atoms it depends upon the phonon
spectrum, and in MSR1a it depends upon the joint atom-electron dielectric
spectrum, specifically the number of eigenvalue bands and their widths.
(Rigorously it depends upon the number and width of the eigenvalues of the
true Jacobian, and also of the current approximation of the Jacobian.)
Insulators converge faster because their spectra are simpler.
When you fix some atoms it does not follow that you simplify the spectra,
in fact you may be creating anomalous dielectric bands. You are also now
doing a constrained optimization, and these are almost always slower and
less well conditioned.
Plus fixing one surface may create a dipole, electronic defect states in
the gap and other non-physical mess.
___
Emeritus Professor Laurence Marks (Laurie)
Department of Materials Science and Engineering, Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi
On Wed, May 7, 2025, 03:49 Laurence Marks <laurence.marks at gmail.com> wrote:
> Depending upon which version is used the center of mass fixing may be
> inactive when atoms are fixed, which is why I asked about that. Plus the
> other questions, as some of the procedure used does not seem right.
>
> ___
> Emeritus Professor Laurence Marks (Laurie)
> Department of Materials Science and Engineering, Northwestern University
> www.numis.northwestern.edu
> "Research is to see what everybody else has seen, and to think what nobody
> else has thought" Albert Szent-Györgyi
>
> On Wed, May 7, 2025, 03:36 Peter Blaha <peter.blaha at tuwien.ac.at> wrote:
>
>> mixer tries to keep the center of mass constant (to avoid that all atoms
>> move in some direction, although their distances are in equilibrium).
>>
>> At the moment, you cannot switch this off, although it would make sense
>> if an atom is constrained in a certain direction (Laurie: I know, you
>> don't like these constraint positions, but it might be a valuable
>> modification to use the center of mass in a certain direction only if no
>> positions are constrained).
>>
>> In general I would NOT recommend to fix only one side of the slab, at
>> least not if you have mirror symmetry along z. You have then a
>> calculation with a relaxed and an unrelaxed surface !
>>
>> What makes sense, is to fix the middle layers of a slab....
>>
>> Am 07.05.2025 um 10:21 schrieb Hadjer MEZIANI via Wien:
>> > Dear wien2k users,
>> > I am performing geometry minimization (MSR1a) of the TiO₂ surface to
>> > relax the top layer while keeping the bottom layers fixed, using
>> > WIEN2k_24.1. I followed these steps:
>> > 1-Generate struct
>> > 2- init-lapw
>> > 3-mini.positions(w2web) and i edit case.inM after that i run pairhase;
>> > copy .minpair to .minrestart and delete .min_hess.
>> > 4- run scf with activate optimize positions(MSR1a) with -fc 20.
>> > I constrained the atoms in the bottom layers by setting `delta=0`, but
>> > these atoms are still displaced during the minimization.
>> >
>> > .inM file in the following way:
>> > PORT 2.00 0.35 2.00 # PORT/NEWT; tolf, Initial Trust Radius, tolfa
>> > 0.0 0.0 0.0 1.0 #Atom 1 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 2 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 3 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 4 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 5 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 6 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 7 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 8 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 9 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 10 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 11 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 12 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 13 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 14 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 15 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 16 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 17 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 18 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 19 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 20 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 21 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 22 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 23 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 24 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 25 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 26 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 27 Generated by pairhess
>> > 1.0 1.0 1.0 1.0 #Atom 28 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 29 Generated by pairhess
>> > 0.0 0.0 0.0 1.0 #Atom 30 Generated by pairhess
>> > For example, atom 1 has changed position even though it was fixed in
>> > `.inM`. See output:
>> > --- atom dependend parameter POS -----------
>> > in 1 files:
>> > :POS001: ATOM -1 X,Y,Z = 0.50000 0.00000 0.04398 MULT= 1 ZZ= 22.000
>> Ti
>> > :POS001: ATOM -1 X,Y,Z = 0.50000 1.00000 0.04397 MULT= 1 ZZ= 22.000
>> Ti
>> > :POS001: ATOM -1 X,Y,Z = 0.50000 1.00000 0.04396 MULT= 1 ZZ= 22.000
>> Ti
>> > :POS001: ATOM -1 X,Y,Z = 0.50000 1.00000 0.04389 MULT= 1 ZZ= 22.000
>> Ti
>> > :POS001: ATOM -1 X,Y,Z = 0.50000 0.00000 0.04382 MULT= 1 ZZ= 22.000
>> Ti
>> > :POS001: ATOM -1 X,Y,Z = 0.50000-0.00000 0.04337 MULT= 1 ZZ= 22.000
>> Ti
>> > ......
>> > My question is: how can i keep positions of these atoms unchanged
>> during
>> > structure minimzation!
>> > Any suggesting are highly welcome
>> > Thank you in advence
>> > M. Hadjer
>> >
>> >
>> >
>> >
>> > _______________________________________________
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>>
>> --
>> -----------------------------------------------------------------------
>> Peter Blaha, Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna
>> Phone: +43-158801165300
>> Email: peter.blaha at tuwien.ac.at
>> WWW: http://www.imc.tuwien.ac.at WIEN2k: http://www.wien2k.at
>> -------------------------------------------------------------------------
>>
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>
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