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

[vtmtrinh at caltech.edu]

Dear Stefano,

Thank you very much for your help.

Trinh

> Dear  Trinh,
> considering that the bulk modulus of silicon is about 1 Mbar (1000 Kbar)
> a convergence error in the calculated pressure of about 3 Kbar implies
> an error in the volume of about 0.3 % which mean 0.1 % for the lattice
> parameter which is what you have and is usually considered a good
> accuracy. Other factors like XC functional and fitting procedure give
> larger variations
> If you want a convergence accuracy better that 0.1 % you need to
> converge the pressure below a fraction of  Kbar.
>
> stefano
>
> PS: as for the difference between the stress calculated by vc-relax at
> the end of the relaxation and an scf calculation done at the converged
> structure this is due to the fact that during vc-relax the calculation
> is performed at fixed number of plane waves so that the two calculations
> are not done with the same basis.
>
>
>
>
> vtmtrinh at caltech.edu wrote:
>> 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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