I found the Meshcutoff was commented. I clear the mistake and rerun the program. I got similar result. Still 3 nontrival frequency. Anybody could explain why?
Quoting [email protected]: > > > 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 >> >> >
