stal axes: (cart. coord. in units of alat)
 -0.5  0.0  0.5
  0.0  0.5  0.5
 -0.5  0.5  0.0

reciprocal axes: (cart. coord. in units 2 pi/alat)
 -1.0 -1.0  1.0
  1.0  1.0  1.0
 -1.0  1.0 -1.0