> 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?
It seems a bit strange that the "Meshcutoff commented" (i.e., default?) produces "similar result" as 500 Ry. Anyway. Apparently as you displace H atom perpendicularly to the H-H distance, the resulting force is not zero, contrary to what could be expected in 1st order over displacements, so you are beyond the harmonic approximation. A possible explanation: the H-H distance is quite small, so the default MD.FCDispl turns out too large. Check its effect. Moreover, what is your basis? - if minimal it can be too bad. Good luck Andrei Postnikov > > 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 >>> >>> >> >
