[Pw_forum] About stress calculation.

[Dariusz Chrobak]

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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