Actually, I changed the Mesh Cut-off to 500 Ry. There are three non-trival frequences. two are near 1033.9, and one is near 3372. And there is no decrease trend when I increase the Mesh-cut off.
Quoting [email protected]: >> 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. > > Probably nothing, but Siesta doesn't know about the symmetry > and produces always 3N frequencies. Of which three must correspond to > uniform translations and be more or less zero, either for crystal or > molecule, and moreover, in general for a molecule, three rotations, > whose frequencies should also be nearly > zero. In case of diatomic molecule however, > one rotation (along the molecule axis) leaves the molecule invariant, > so it should not be counted among these "trivial" modes, and you > stay instead with one "genuine" vibration mode. > It is relatively easy to get zero > frequency of three translation modes (setting MeshCutoff high enough); > the rotational modes are more tricky - I think, it is simply more > numerically demanding. Probably you got them not close enough to zero, > due to unsufficient isotropy of "Siesta space" (mesh, basis etc.). > > Best regards > > Andrei Postnikov > >
h2-siesta.fdf
Description: application/vnd.fdf
h2.fdf
Description: application/vnd.fdf
FC.fdf
Description: application/vnd.fdf
H2.bands
Description: Binary data
H2.vectors
Description: Binary data
