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

[Xunlei Ding]

Dear Zhang,

The tr2_ph is too large in you case.
Just change the tr2_ph=1.0d-14, you will get:
    omega( 1) =       2.198936 [THz] =      73.349094 [cm-1]
     omega( 2) =       3.192379 [THz] =     106.487008 [cm-1]
     omega( 3) =       3.192379 [THz] =     106.487008 [cm-1]
     omega( 4) =       4.915826 [THz] =     163.975388 [cm-1]
     omega( 5) =       4.915826 [THz] =     163.975388 [cm-1]
     omega( 6) =     131.771213 [THz] =    4395.443762 [cm-1]

Yours,
Ding

Jin Zhang wrote:

> Hi all,
>
>    I posted this issue one week ago. Now I have relaxed the structure 
> and got total force less than 5.0D-5. I used a large cubic supercell 
> with 30bohr alat and ecutwfc up to 50Ry. However, things only went 
> worse. The highest frequency from ph.x dropped from 3187 cm^(-1) to 
> 1849, while gaussian03 shows it should be around 3300cm^(-1).
>
> I also checked H2 molecule. In this case, I got three negative values 
> and three positive ones( values are 898,898,3679cm^-1 ). But there 
> should be only ONE vibrational frequency which corresponds to the 
> stretching mode along bonding direction. And the highest freq. is also 
> too low (should be around 43xx).
>
> I really couldn't figure out what's wrong. Can someone help me? Thanks 
> in advance!
>
> The input files for H2 are:
> ---------------------------scf_input-----------------(run two times 
> just like example07 )-----
>  &control
>     calculation='scf'
>     restart_mode='from_scratch',
>     prefix='h2',
>     pseudo_dir = '$PSEUDO_DIR/',
>     outdir='$TMP_DIR/'
>  /
>  &system
>     ibrav=  8, celldm(1) =18, celldm(2)= 1.0, celldm(3)=1.0,
>     nat= 2, ntyp= 1,
>     ecutwfc = 50.0,  
>     occupations='smearing', smearing='gausssian', degauss=0.01,
>     la2F = .false., nspin=1,
>  /
>  &electrons
>     conv_thr =  1.0d-8
>     mixing_beta = 0.7
>  /
> ATOMIC_SPECIES
>   H     1.008    H.blyp-vbc.UPF
> ATOMIC_POSITIONS (bohr)
> H        0.000000000   0.000000000  -0.707244771
> H        0.000000000   0.000000000   0.707244771
> K_POINTS {automatic}
> 1 1 1 0 0 0
> ------------------------------ph_input---------------------------
>  &inputph
>   tr2_ph=1.0d-8,
>   prefix='h2',
>   fildvscf='h2dv',
>   amass(1)=1.008,
>   outdir='$TMP_DIR/',
>   fildyn=' h2.dyn',
>   elph=.false.,
>   trans=.true.,
>   ldisp=.true.,
>   nq1=1, nq2=1, nq3=1
> --------------------------------------------------------------------
>
> Regards,
> Jin Zhang
>