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

degironc degironc at sissa.it
Mon Nov 20 11:25:07 CET 2006


did you relaxed the structure ?
These are the forces I get with your input  [a typical threshold for a 
structural relaxation is
forc_conv_thr  REAL ( default = 1.0D-3 ) ].
stefano

    Forces acting on atoms (Ry/au):

     atom   1 type  2   force =    -0.06870568    0.00011605    0.00070580
     atom   2 type  1   force =     0.00057171   -0.00133033    0.00026104
     atom   3 type  2   force =    -0.03470840    0.06008037    0.00060235
     atom   4 type  1   force =    -0.00200170   -0.00052894   -0.00023669
     atom   5 type  2   force =     0.03477692    0.05989867   -0.00004091
     atom   6 type  1   force =     0.00029793    0.00027534   -0.00032982
     atom   7 type  2   force =     0.06862846   -0.00002844   -0.00054894
     atom   8 type  1   force =    -0.00256825    0.00128897   -0.00025706
     atom   9 type  2   force =     0.03495275   -0.06050673   -0.00040942
     atom  10 type  1   force =    -0.00020204    0.00131722    0.00025463
     atom  11 type  2   force =    -0.03450037   -0.05925204    0.00020053
     atom  12 type  1   force =     0.00345864   -0.00133014   -0.00020152

     Total force =     0.169286     Total SCF correction =     0.000107

stefano


Jin Zhang wrote:

> 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 
> <mailto: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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