[Pw_forum] vc-relax versus scf calculations on various cells

[Stefano de Gironcoli]

Dear Trinh Vo,
     this has been explained a number of times in this forum.
     The way stress is calculated assumes that the number on plane 
waves  does not change  during the deformation.
     This is NOT what you are doing when calculating energy vs alat 
curves at fixed CUTOFF ... which explains the difference between the two 
results.
     Stress converges slowly with cutoff. To be reliable calculations 
involving stress should use higher cutoff than usual.

     Have a look at the attached slides that try to clarify the issue.
     These were presented in a Summer School in Santa Barbara in 2009 
whose material can be accessed at
     http://media.quantum-espresso.org/santa_barbara_2009_07/index.php

  best
   stefano

vtmtrinh at caltech.edu wrote:
> Dear All,
>
> I am running a test calculation on a cell with 64 Si atoms.  I first run a
> series of scf calculations of different cell sizes (the atoms position
> coordinates are also generated accordingly with cell dimensions by my
> code).  I got the following results:
>
> a (A)             Total E(Ry)
> 10.775	        -508.1653034
> 10.770	        -508.1653391
> 10.76932	-508.1653368
> 10.765	        -508.1653067
> 10.76	        -508.1652012
>
> From the above results, it looks like to me I would have a min E close
> to/at a=10.77A (plotting E vs a give me a nice parabola curve).  I then
> rerun the structure at min E (i.e. a=10.77) using vc-relax.  I obtained
> the following:
>
> CELL_PARAMETERS (alat)
>    1.000000000   0.000000000   0.000000000
>    0.000000000   1.000000000   0.000000000
>    0.000000000   0.000000000   1.000000000
>
>
> alat = 20.3289 au (or 10.75760138 A),  E=-508.1650690 Ry
>
> Comparing to the results by scf calculations, I see the result from
> vc-relax and scf calculations are not consistent at all.  I thought they
> should. However, the total stress computed for the scf calculation at min
> energy is still large (P = -3.36 kbar), while the stress calculated by the
> vc relax is small  P = -0.04 kbar.  I also noted that the cell size at
> which the system has a min stress (i.e. almost zero stress) given by scf
> calculations does not coincide with the cell size at which the the system
> has a minimum energy (min of the parabola).  I thought that at the
> optimized cell size, both stress and energy have to be minimum. I don't
> understand why there is such a difference.  Am I wrong in the way I was
> doing?
>
> Thank you very much,
>
> Trinh Vo
> JPL/Caltech
>
>
>
>
>
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>   

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