[Pw_forum] lattice constant from unit cell vs. from supercell

[Stefano de Gironcoli]

dear Chad Junkermeier,
if two calculations are equivalent the code gives the same result.

your calculation at alat = 4.7 has
Final energy = -71.3700391584 Ry in the 1x1 cell and
Final energy = -285.4177075378 Ry = -71.35442688 Ry x 4  in  the 2x2 cell.
This difference is significant,  the two systems are not the same....

Check your positions, your k-point sampling, your cutoff etc ...
hope this helps,
 stefano

Chad Junkermeier wrote:
> Yes, from what I wrote in my last post I can see the confusion.
>
> In my last post I sent two representative files.  In actuality, I  
> performed a series of calculations, each with a different alat value,  
> on the unit cell.  The calculation that had the lowest "Final energy"  
> was the was the one that I say has the equilibrium lattice constant  
> for the unit cell.  I give here a listing of some of the results:
>
> alat = 4.7
>       Final energy   =     -71.3700391584 Ry
> alat = 4.750
>       Final energy   =     -71.3737078425 Ry
> alat = 4.800                                                       
> <---- Equilibrium value
>       Final energy   =     -71.3753602156 Ry
> alat = 4.850
>       Final energy   =     -71.3751659760 Ry
> alat = 4.900
>       Final energy   =     -71.3732897789 Ry
> alat = 4.950
>       Final energy   =     -71.3698878408 Ry
>
>
> I also performed a series of calculations, each with a different alat  
> value, on the 2x2 supercell.  Of those calculations, the one with the  
> lowest "Final energy" is the one that has the equilibrium lattice  
> constant for the supercell case.  Here are some of the results for  
> this case:
>
> glat = 4.55
>       Final energy   =    -285.4093082597 Ry
> glat = 4.6
>       Final energy   =    -285.4217551819 Ry
> glat = 4.625
>       Final energy   =    -285.4242025245 Ry
> glat = 4.65                                                      <----  
> Equilibrium value
>       Final energy   =    -285.4242932895 Ry
> glat = 4.675
>       Final energy   =    -285.4221256935 Ry
> glat = 4.700
>       Final energy   =    -285.4177075378 Ry
> glat = 4.725
>       Final energy   =    -285.4115070010 Ry
>
>
>
> Since the supercell case is essentially the same system as the unit  
> cell case, the equilibrium value for alat should be the same in both  
> cases.  I am finding that they are not.  Further, the final positions  
> of the unit cell case has atom positions near where we think they  
> should be, but in the 2x2 unit cell the final atom positions are not.
>
>
>
> Chad Junkermeier, Ph.D.
> NRC Post-Doctoral Fellow
> Naval Research Laboratory
> 4555 Overlook Ave, S.W.
> Washington DC 20375
> Office: (202) 404-4635
> Fax: (202) 767-1165
>
>
>
>
>
>
>
> On Mar 17, 2010, at 12:52 PM, Paolo Giannozzi wrote:
>
>   
>> Chad Junkermeier wrote:
>>
>>     
>>> If I do this using the unit cell, everything seems to
>>> work correctly, and the equilibrium lattice constant for the system
>>> seems to be correct.  If, instead, I use a 2x2 supercell I find that
>>> the equilibrium lattice constant is not the same (or even close) to  
>>> the
>>> unit cell result.
>>>       
>> your data are for fixed-cell structural optimization: what do you mean
>> by "the equilibrium lattice constant is not the same"? You choose the
>> lattice constant
>>
>> P.
>> -- 
>> Paolo Giannozzi, Democritos and University of Udine, Italy
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>
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