[Pw_forum] About stress calculation.

[Dariusz Chrobak]

Uz.ytkownik CLAUDIA BUNGARO napisa?:

>Dear Dariusz,
>for Ni2MnGa the shear elastic constants are also sensitive
>upon the BZ sampling. A denser k-point mesh is needed to obtain
>converged values at a level of a few GPa (for details see Bungaro et al.
>PRB 68, 134104 (2003) section III.D).
>
>Best,
>Claudia
>
>__________________________________________________
>Dr. Claudia Bungaro
>Dept. of Physics and Astronomy, Rutgers University
>136 Frelinghuysen Road, Piscataway,  NJ 08854-8019
>
>tel: +1 732 445 4197          fax: +1 732 445 4343
>email: bungaro at physics.rutgers.edu
>__________________________________________________
>
>
>On Tue, 1 Mar 2005, Dariusz Chrobak wrote:
>
>  
>
>>Dear users,
>>
>>I have a problem with stress calculation for Ni2MnGa (L21 structure). I
>>am using pseudopotential from PWscf site.
>>The tetragonal distorsions  c/a=0.995 and c/a=1.005
>>(epsilon_xx=epsilon_yy=e, epsilon_zz=-2e where c/a=(1-2e)/(1+e)) were
>>aplied to the regular cell with a0=10.9718 a.u..
>>
>>The stress calculation performed for these cases gave me the folowing
>>results:
>>
>>1. c/a=0.995
>>          total   stress  (ryd/bohr**3)                  (kbar)
>>P=    0.00
>>   0.00000328   0.00000000   0.00000000          0.48      0.00      0.00
>>   0.00000000   0.00000328   0.00000000          0.00      0.48      0.00
>>   0.00000000   0.00000000  -0.00000656          0.00      0.00     -0.97
>>2. c/a=1
>>          total   stress  (ryd/bohr**3)                  (kbar)
>>P=    0.00
>>  -0.00000003   0.00000000   0.00000000          0.00      0.00      0.00
>>   0.00000000  -0.00000003   0.00000000          0.00      0.00      0.00
>>   0.00000000   0.00000000  -0.00000003          0.00      0.00      0.00
>>3. ca=1.005
>>          total   stress  (ryd/bohr**3)                  (kbar)     P=
>>-0.22
>>   0.00000093   0.00000000   0.00000000          0.14      0.00      0.00
>>   0.00000000   0.00000093   0.00000000          0.00      0.14      0.00
>>   0.00000000   0.00000000  -0.00000634          0.00      0.00     -0.93
>>
>>So, my problems are:
>>1. The elastic constant c'=(c11-c2)/2 (sigma_xx=sigma_yy=2c'e)
>>calctuated from stresses are different.
>>2. Sigma_ij should change sign if c/a changes from  0.995 to 1.005.
>>
>>Please, help me find what I am doig wrong.
>>
>>Best regards
>>Dariusz Chrobak
>>
>>    
>>
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>
>  
>
Dear Claudia and Stefano,

Thank you for reply.

Best regards,
Dariusz