[Pw_forum] Confused about vibrational modes of isolated molecule from ph.x

[Miguel Martínez Canales]

Hi Jin,

Jin Zhang escribió:
> The thing I cannot understand is, there're 12 negative frequencies 
> existing. I know 3 of them are purely translational modes and another 3 
> are rotational ones. But what about the rest 6? I'm a little confused 
> and hopefully someone can help.
> 
> The eigenvals are pasted below and thanks in advance!
> 
> (the result may not be accurate due to a only-15Ry ecut I used, but it 
> should not give out qualitative different result)

For a start, I wouldn't think 15 rydbergs is enough. Have you tested the 
pseudopotentials for ecut convergence? 25 or 30 would probably be a better 
choice. Anyway, what you probably are missing most in your calculations is 
k-point sampling convergence. Translational and rotational frequencies are 
a bit too large. What happens if you increase your k-point grid? Anyway, 
don¡t forget to relax your molecule with the new parametres.

BTW: What kind of cell are you using? I've never done molecular normal mode 
calculations but I guess that in the case of bencene using an hexagonal 
supercell might make sense.

-- 
----------------------------------------
Miguel Martínez Canales
    Dto. Física de la Materia Condensada
    UPV/EHU
    Facultad de Ciencia y Tecnología
    Apdo. 644
    48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
----------------------------------------

  "The problem with Renault is that they dont have
  the skills to convince FIA that the mass damper
  is just a brake cooler."

Annonymous