[Pw_forum] Confused about vibrational modes of isolated molecule from ph.x
Jin Zhang
jin.zhang.pku at gmail.com
Sun Nov 19 14:42:03 CET 2006
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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