[Pw_forum] how to do fixed volume optimization

[Kun Yin]

Hi, Han

It is well known that GGA functionals tend to overestimate lattice parameters, while LDAs underestimate. When performing phonon calculations, using the experimental volume of a crystal will give more accurate frequencies with experimental data than using theoretically predicted volume. So I need this functionality when doing "the volume scaling" computations.

But I have a new idea.  Maybe this could be done by replacement of "the volume scaling" by "pressure scaling" through an equation of state. 

------------------				 
Kun Yin
2010-06-01

Hello Kun Yin,

I'd appreciate if you would let me know why constant-volume vc-relax is
"very useful" in high-pressure physics.

Here is my response to your question. Do you really need to perform
vc-relax at a "constant" volume, or do you just need the final volume to
be very close to the initial volume? If the latter, you can do so by
properly setting the target pressure (press), usually through good
guessing, in your input file.

Han

> I see. Thanks for giving the answer, Paolo. I hope this easy-to-implement
> functionality, doing 'vc-relax' at constant volume, will be done by some
> enthusiastic guy, since it is very useful in some research such as the
> study of high-pressure materials.
> Kun Yin
> 2010-05-31
>
>

>
> short answer: no. It should be relatively easy to implement but
> it has never been done, to the best of my knowledge. You can
> use 'vc-relax' at costant pressure, though, and vary the
> pressure, if I remember correctly
>
> P.
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy

*******************************************************
Han Hsu, Ph.D.
Postdoctoral Associate
Department of Chemical Engineering & Materials Science
University of Minnesota
http://www.cems.umn.edu/~hsuhan
*******************************************************


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