[Pw_forum] d3.x: Symmetry operation in third-order anharmonic force tensor of Si

[Lorenzo Paulatto]

In data 25 gennaio 2011 alle ore 06:13:48, Haru Deku  
<adadadad3018 at gmail.com> ha scritto:
> I am a really beginner for use of d3.x, and may be this problem comes  
> from a lack of my attentiveness to the calculation.

Dear Haru,
the d3 code has not been used very often, I'm pretty sure it contains  
quite a few bugs and behaves in unexpected ways from times to times.  
However, I'm not sure you there is a problem in your case.

I can see you have 6 modes, that thanks to symmetry are 2-fold degenerate.  
Hence you can mix together the lines and columns of the D3 tensor that  
correspond to degenerate modes without changing any of the observable  
quantities that can be derived from it.

If you check the output of the D3 calculation you'll see that the  
displacement patter used in the (0,0,1) and in the (0,1,0) calculation are  
indeed very different, albeit equivalent. Please not that the same  
"problem" exists for the phonon calculation, but the D2 matrix has a  
pretty obvious and simply related observable (the eigenvalues) that you  
can promptly compare and is gauge invariant.

best regards


-- 
Lorenzo Paulatto
post-doc @ IMPMC/UPMC - Université Paris 6
phone: +33 (0)1 44 27 74 89
www:   http://www-int.impmc.upmc.fr/~paulatto/

previously (take note of the change!):
phd student @ SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
www:   http://people.sissa.it/~paulatto/