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

[vtmtrinh at caltech.edu]

Dear Stefano,

I have read the link you show.  Thank you very much.  I actually used high
Ecut (35Ry).  The results I sent in the previous email are for Ecut-35Ry. 
Let me check everything again to see if I made any mistake.

Best,

Trinh

> 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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