[Pw_forum] About stress calculation.

[CLAUDIA BUNGARO]

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
>