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

[Jin Zhang]

Dear MMC,

The supercell I used is a simple cubic one (let ibrav=8
celldim(1,2,3)=20bohr). Because the molecule is isolated, I think the kpoint
sampling should be irrelevant. And I used gamma point for the calculation.

I increased ecut to 25Ry, and celldim to 30bohr) the number of negative
modes increase by 9 (to 21modes)!
OMG, there must be sth wrong!
I'll attach my input and see if someone can give some comments.
!-----------------------omega-------------------
     omega( 1) =    -123.660534 [THz] =   -4124.898815 [cm-1]
     omega( 2) =    -122.070631 [THz] =   -4071.865015 [cm-1]
     omega( 3) =    -109.408034 [THz] =   -3649.483445 [cm-1]
     omega( 4) =    -103.611190 [THz] =   -3456.120267 [cm-1]
     omega( 5) =    -100.288151 [THz] =   -3345.274863 [cm-1]
     omega( 6) =     -86.921490 [THz] =   -2899.408087 [cm-1]
     omega( 7) =     -86.648147 [THz] =   -2890.290289 [cm-1]
     omega( 8) =     -78.536226 [THz] =   -2619.703943 [cm-1]
     omega( 9) =     -63.552993 [THz] =   -2119.913758 [cm-1]
     omega(10) =     -60.437624 [THz] =   -2015.995524 [cm-1]
     omega(11) =     -59.290658 [THz] =   -1977.736621 [cm-1]
     omega(12) =     -59.022664 [THz] =   -1968.797246 [cm-1]
     omega(13) =     -58.953578 [THz] =   -1966.492750 [cm-1]
     omega(14) =     -46.326804 [THz] =   -1545.306104 [cm-1]
     omega(15) =     -31.263942 [THz] =   -1042.859776 [cm-1]
     omega(16) =     -31.093207 [THz] =   -1037.164629 [cm-1]
     omega(17) =     -27.758798 [THz] =    -925.939988 [cm-1]
     omega(18) =     -27.694407 [THz] =    -923.792135 [cm-1]
     omega(19) =     -24.097859 [THz] =    -803.823410 [cm-1]
     omega(20) =     -20.675736 [THz] =    -689.672916 [cm-1]
     omega(21) =     -20.064562 [THz] =    -669.286187 [cm-1]
     omega(22) =      15.218497 [THz] =     507.637805 [cm-1]
     omega(23) =      16.277849 [THz] =     542.974191 [cm-1]
     omega(24) =      20.249558 [THz] =     675.457041 [cm-1]
     omega(25) =      21.320279 [THz] =     711.172668 [cm-1]
     omega(26) =      22.104349 [THz] =     737.326626 [cm-1]
     omega(27) =      23.427206 [THz] =     781.452687 [cm-1]
     omega(28) =      30.392570 [THz] =    1013.793763 [cm-1]
     omega(29) =      43.644315 [THz] =    1455.827328 [cm-1]
     omega(30) =      55.006552 [THz] =    1834.833267 [cm-1]
     omega(31) =      57.997854 [THz] =    1934.613035 [cm-1]
     omega(32) =      61.898591 [THz] =    2064.728462 [cm-1]
     omega(33) =      68.862483 [THz] =    2297.020463 [cm-1]
     omega(34) =      68.979827 [THz] =    2300.934652 [cm-1]
     omega(35) =      69.303015 [THz] =    2311.715098 [cm-1]
     omega(36) =      69.697807 [THz] =    2324.884051 [cm-1]

On 11/19/06, Miguel Martínez Canales < wmbmacam at lg.ehu.es> wrote:
>
> 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
>
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-- 
Jin Zhang
Dep. of Physics,
Peking Univ.
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