[Wien] Forces using Tetra

[Laurence Marks]

Sorry, I do not know a paper.

My understanding of this issue is incomplete as I have not been able
to formulate it "properly". I suspect that he was worried about the
derivatives of the partial occupancies with position, which are not
going to be exactly right except in the limit of a large number of
k-points. This is going to introduce some "noise" (I think similar to
telegraph noise) into the forces -- and in fact TETRA does do this in
general, and this noise goes to zero as the number of k-points
increases. Hence a common alternative is to use TEMPS which is the
Mermin functional and variationaly correct (note that TEMP is not
variationaly correct). The downside is that you need more k-points for
convergence, but it tends to be (in my experience) less noisy. While
it is "common" to use large numbers for the effective temperature
(e.g. 0.01 Rydberg, ~1500K) to improve convergence in my experience
room temperature (0.0018) is fine. (Many people say this has no
physical significance, but I don't understand why they say this.)

2011/7/28 David Tompsett <dat36 at cam.ac.uk>:
> Hi Laurence,
>
> Is there any explanation or reference as to why they are OK? The Bloch paper
> suggests that they won't be.
>
> Cheers,
> David.
>
> 2011/7/28 Laurence Marks <L-marks at northwestern.edu>
>>
>> The forces are fine.
>>
>> On Jul 28, 2011 4:55 AM, "David Tompsett" <dat36 at cam.ac.uk> wrote:
>> > Dear All,
>> >
>> > I have a question about the forces obtained when using TETRA in metallic
>> > systems. I read in the 1994 Blochl paper that the Blochl Tetra method is
>> > not
>> > variational with respect to partial occupancies. Does this mean that the
>> > total energy from Tetra is not smoothly variational wrt. to partial
>> > occupancies?
>> >
>> > Therefore, can I trust the calculated forces in a metallic system?
>> >
>> > Can I use the total energy from Tetra for metals reliably?
>> >
>> > Many thanks for any help,
>> > David Tompsett.
>>
>> _______________________________________________
>> Wien mailing list
>> Wien at zeus.theochem.tuwien.ac.at
>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>>
>
>
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>
>



-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Research is to see what everybody else has seen, and to think what
nobody else has thought
Albert Szent-Gyorgi