[Pw_forum] a question about how to choose the best relaxed structure

[Sandro Scandolo]

Dear Xu Yuehua,

the calculated stress tensor is not exactly equal to the derivative of 
the energy if the basis set is not complete. For a detailed discussion 
see: http://www.quantum-espresso.org/wiki/index.php/Stress_calculations

Regards,
Sandro
 
xu yuehua wrote:
> hi everyone here :
>  i have a puzzled question about the relaxed structure :
> i relaxed a structure : ,i set c direction of crystal lattice is 
> 2.7950 and the final result is
>
>  entering subroutine stress ...
>
>           total   stress  (Ry/bohr**3)                   (kbar)     
> P=   -1.06
>   -0.00000893  -0.00000001  -0.00000001         -1.31      0.00      0.00
>   -0.00000001  -0.00000892   0.00000000          0.00     - 1.31      0.00
>   -0.00000001   0.00000000  -0.00000382          0.00      0.00     -0.56
>
>
>      bfgs converged in  25 scf cycles and  22 bfgs steps
>
>      End of BFGS Geometry Optimization
>
>      Final energy =    -137.7926586085 Ry
>
> so , i reduced the length of c ,set to 2.760, and the result is :
>
>  entering subroutine stress ...
>
>           total   stress  (Ry/bohr**3)                   (kbar)     
> P=   -0.81
>   -0.00000882  -0.00000004   0.00000001         -1.30     -0.01      0.00
>   -0.00000004  -0.00000881   0.00000000         -0.01     - 1.30      0.00
>    0.00000001   0.00000000   0.00000106          0.00      0.00      0.16
>
>
>      bfgs converged in  35 scf cycles and  32 bfgs steps
>
>      End of BFGS Geometry Optimization
>
>      Final energy =    -137.7921913584 Ry
>
> and  my problem is which the structure is better ?
> according to stress ,seems latter,but accrording  the energy ,seems 
> the  former ?
> i do not know
> so please give me some help
> thank you
>  
>  
>
> -- 
> Xu Yuehua
> physics Department of Nanjing university
> China
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