[Wien] Structure optimization

[Laurence Marks]

It works...but (sorry Peter) it can be bad science. There are special cases
where it is appropriate, but it can also be very inappropriate.

There is a long history of people fixing some atomic positions to bulk
positions when performing surface calculations. However, the strain field
of any surface decays exponentially in the selvedge layer, and removing
this can be very wrong. For instance, there are many known cases where the
surface induces a ferroelectric distortion which fixed positions will
prevent from happening. The rigorous method is to check that the atomic
positions in the center of the slab are "close enough" to those in the
bulk. If they are not then either the slab needs to be thicker or there has
been a surface induced phase transition.

And, fixing positions for a surface *in general does not improve the
convergence*. If you are lucky it will, but not in general. The reason (as
I have mentioned before) is that the convergence scales with the number and
width of the eigenvalue clusters, not the number of variables. If the
convergence of Wien2k and other DFT codes scaled with the number of density
variables, the methods would be useless! DFT is a useful method because it
will typically converge in 10-100 scf iterations because there are only a
few eigenvector clusters of the dielectric response (in density only
calculations) and the force matrix (with PORT). MSR1a is typically
efficient because the number and width of the clusters of the general
dielectric response with both density and electronic terms included is
smaller than the product of the clusters for the electronic and atomic
components.

Sorry for being so pedantic...but rigor matters.

On Sat, May 19, 2018 at 7:56 AM, Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> > - How can I lock some atoms in position during structure optimization?
>
> In case.inM  put the 1.0 values of x,y and/or z of the corresponding
> atom (line) to 0.0.
>
> It works well. We do it all the time....
>
> >
> > Best,
> > Lukasz
> >
>
> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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-- 
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
Co-Editor, Acta Cryst A
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