primitive cell
 1.0000000
     -2.733099421887      0.000000000000      2.733099421887
      0.000000000000      2.733099421887      2.733099421887
     -2.733099421887      2.733099421887      0.000000000000
Si 
  2
Direct
     0.250000000000000000     0.250000000000000000     0.250000000000000000
     0.500000000000000000     0.500000000000000000     0.500000000000000000
