[SIESTA-L] Problem about calculation of vibration mode of H2

[Zhen Zhu]

I am trying to calculate the vibration mode of H2 molecure. I put H2 
into a unit cell of a lattice, and then make the lattice constant equal 
to 10 Ang ( I think this will prevent the interaction of different H2 
molecure in different unit cell). However, after following the way ( 
using fcbuild to make FC.fdf and using siesta to calculate FC constant 
and then analyze using vibrator), I get 6 vibration mode, while 3 are 
trivial, but the other three are not). As H2 is a linear molecure, I 
think the number of vibration mode should be 1. I am not sure what's 
wrong with my calculation. Someone could help me?

Thanks