Si possesses a face-centered cubic structure with two equivalent
atoms per unit cell, at (0.125,
0.125,
0.125).
The site symmetry is -43m. For the
-phnon the two atoms
are displaced in opposite direction along the (111) direction
and cubic symmetry is lost. The output of SYMMETRY gives the
following information:
Si operation # 1 1 Si operation # 13 m n -110 Si operation # 16 m n -101 Si operation # 17 m n 0-11 Si operation # 24 3 || 111 Si operation # 38 3 || 111 pointgroup is 3m (neg. iatnr!!) axis should be: 3 || z, m n y lm: 0 0 1 0 2 0 3 0 3 3 4 0 4 3 5 0 5 3 6 0 6 3 6
Therefore the required local rotation matrix should rotate z into the (111) direction and thus the matrix in the ``struct`` file should be:
0.4082483 -.7071068 0.5773503 |
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0.4082483 0.7071068 0.5773503 |
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-.8164966 0.0000000 0.5773503 |
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