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

[Stefano de Gironcoli]

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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