[Wien] Question on convergence of charged state

[Laurence Marks]

My response about metals and MSR1a was short, so for others as well let me
clarify.

The PORT optimizer works on the Born-Oppenheimer surface, I.e. it uses the
forces for a converged density. MSR1a simultaneously optimizes the
positions and densities. There will be a change in the density drho due to
a change in an atomic position of dx. In MSR1a the convergence of the
density will be about drho when the atomic positions have converged to dx.
One way to thing about this is via C=drho/dx.

For insulators C is typically small, there are no free states at the Fermi
energy. For metals C is typically much larger, and by metal I mean anything
with many unoccupied states at the Fermi energy. Hence for insulators the
density converges better than it does with metals.

And, to confuse everything further, MSR1a is not really using the forces
that appear at the bottom of case.scfm (case.scf). These are the sum of two
terms, the true forces F_t and ones due to the density not being converged
F_nc. The later go to zero when the density converges, so are larger for
metals. MSR1a uses simplex gradients, and can somewhat separate the two --
if it did not it would not work. My observation is that for metals F_t is
often much smaller than F_nc near the solution, so switching from MSR1a to
MSR1 when the displacements are small (converged positions) leads to a drop
in the forces.

Unfortunately we don't currently have a good independent method of
separating F_t & F_nc, so the knowing when MSR1a should be stopped is an
unsolved problem.

---
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 Feb 23, 2016 06:45, "Laurence Marks" <laurence.marks at gmail.com> wrote:

> OK, I guess I did do MSR1a+charge.
>
> I don't think there is an issue, just switch to MSR1. For insulators MSR1a
> often converges the charge & forces very well. For metals a -cc of 0.001 is
> more normal, and you probably have a metal (degenerate semiconductor) due
> to the charge.
>
> ---
> 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 Feb 22, 2016 22:21, "Hu, Wenhao" <wenhao-hu at uiowa.edu> wrote:
>
>> Hi, Professor Marks:
>>
>> The following is the results from the label of :NEC:
>>
>> :NEC03: NUCLEAR AND ELECTRONIC CHARGE    814.00000   813.99598 Valence
>> :NEC01: NUCLEAR AND ELECTRONIC CHARGE    814.00000   813.96208
>> :NEC02: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00000
>> :NEC03: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00092 Valence
>> :NEC01: NUCLEAR AND ELECTRONIC CHARGE    814.00000   813.96207
>> :NEC02: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00000
>> :NEC03: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00091 Valence
>> :NEC01: NUCLEAR AND ELECTRONIC CHARGE    814.00000   813.96207
>> :NEC02: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00000
>> :NEC03: NUCLEAR AND ELECTRONIC CHARGE    814.00000   814.00063 Valence
>>
>> About the charged state of defects in diamond, transitions among
>> different charged states are possible to show up depending on the location
>> of the Fermi energy. You can think about the nitrogen vacancy center, which
>> has two stable states, i.e. neutral and -1 charged. The latter is proved to
>> exhibit a spin one ground state.  Even though the supercell calculations
>> have some flaws, my calculations so far qualitatively show a similar result
>> in some charged isolated vacancy depending on various factors. The charged
>> electron should be delocalized, but I can’t confirm the extent without a
>> converged results. The thing I can confirm is that the magnetic moment are
>> all from the defect level. Or maybe I didn’t get your question very well.
>> If any other information about my calculation is needed, please let me know.
>>
>> Thank you very much for your help. I really appreciate it.
>>
>> Wenhao
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>
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