[Pw_forum] Frequency and zero point energy calculations

[Gabriele Sclauzero]

Dear Lihui Ou,

oulihui666 wrote:
> Dear all,
>     Recently, I have been performing calulation of frequency and zero 
> point energy of CO2 molecule. However, unfortunately, I have 
> obtained negative frequency. According to the manual, there are several 

>                        ibrav = 1,
>                    celldm(1) = 15.11781502,

This seems too small. You want to avoid spurious interactions between periodic replicas of 
the molecule.

>                          nat = 3,
>                         ntyp = 2,
>                      ecutwfc = 26 ,
>                      ecutrho = 260 ,

These cut-offs may also be too small. C and O have 2p electrons which are hard to deal 
with PWs, even with US PPs.

>                  occupations = 'smearing' ,
>                      degauss = 0.02 ,

This is too large for a molecule, you can use much smaller values.

>  &ELECTRONS
>                     conv_thr = 1.D-12 ,

This requirement may be too strict. Anyway, if you don't have problems in convergence you 
can keep it as it is.

> (2) Performing scf calculations:
>  &CONTROL
>                  calculation = 'scf' ,
>                 restart_mode = 'from_scratch' ,
>                       outdir = '/home/olh/tmp/' ,
>                   pseudo_dir = '/home/olh/pseudo/' ,
>                       prefix = 'CO2' ,
>  /
>  &SYSTEM
>                        ibrav = 1,
>                    celldm(1) = 15.11781502,
>                          nat = 3,
>                         ntyp = 2,
>                      ecutwfc = 26 ,
>                      ecutrho = 260 ,
>                  occupations = 'smearing' ,
>                      degauss = 0.02 ,
>                     smearing = 'methfessel-paxton' ,
>                        nspin = 2 ,
>    starting_magnetization(1) = 0.5,
>    starting_magnetization(2) = 0.5,
>  /
>  &ELECTRONS
>                     conv_thr = 1.D-12 ,
>                  mixing_beta = 0.4D0 ,
>  /
> ATOMIC_SPECIES
>     C   12.01100  C.pbe-rrkjus.UPF
>     O   15.99940  O.pbe-rrkjus.UPF
> ATOMIC_POSITIONS angstrom
>     C        4.000000000   4.000000000   4.000000000    0  0  0
>     O        5.172018724   4.000000000   4.000000000    0  0  0
>     O        2.827981276   4.000000000   4.000000000    0  0  0
> K_POINTS automatic
>   16 16 16  1 1 1

Maybe I don't understand what you want to calculate, but I don't see any need of a k-point 
sampling when computing an isolated molecule. Why don't you use gamma as in the relax 
calculation?


>  
> (3) Performing frequency calculation using PHONOS codes:
> phonons of CO2 at Gamma
>  &inputph
>   tr2_ph=1.0d-20,

This is very strict!

>   prefix='CO2',
>   epsil=.false.,
>   amass(1)=12.0110,
>   amass(2)=15.9994,
>   outdir='/home/olh/tmp/',
>   fildyn='CO2.dynG',
>  /
> 0.0 0.0 0.0



> Finally, I obtained the frequency of CO2 molecule, output file as follows:
>     Convergence has been achieved
>     Number of q in the star =    1
>     List of q in the star:
>          1   0.000000000   0.000000000   0.000000000
>     Diagonalizing the dynamical matrix
>     q = (    0.000000000   0.000000000   0.000000000 )
> **************************************************************************
>     omega( 1) =      -3.661345 [THz] =    -122.130124 [cm-1]
>     omega( 2) =      -3.661345 [THz] =    -122.130124 [cm-1]
>     omega( 3) =       3.659957 [THz] =     122.083850 [cm-1]
>     omega( 4) =       7.879986 [THz] =     262.849774 [cm-1]
>     omega( 5) =       7.879986 [THz] =     262.849774 [cm-1]
>     omega( 6) =      19.746862 [THz] =     658.688787 [cm-1]
>     omega( 7) =      19.746862 [THz] =     658.688787 [cm-1]
>     omega( 8) =      38.945041 [THz] =    1299.075383 [cm-1]
>     omega( 9) =      70.000609 [THz] =    2334.984491 [cm-1]
> **************************************************************************
>      Mode symmetry, D_4h(4/mmm) point group:
>      omega(  1 -  2) =       -122.1  [cm-1]   --> E_g  X_5  M_5   R
>      omega(  3 -  3) =        122.1  [cm-1]   --> A_2u X_4' M_4'  I
>      omega(  4 -  5) =        262.8  [cm-1]   --> E_u  X_5' M_5'  I
>      omega(  6 -  7) =        658.7  [cm-1]   --> E_u  X_5' M_5'  I
>      omega(  8 -  8) =       1299.1  [cm-1]   --> A_1g X_1  M_1   R
>      omega(  9 -  9) =       2335.0  [cm-1]   --> A_2u X_4' M_4'  I
>  **************************************************************************
>      PHONON       :     1h48m CPU time,        2h25m wall time
> ......
> There are still negative frequency in output file by increasing K_POINTS 
> and reducing tr2_ph, anyone who help me will be appreciated.

Increasing the number of k-points won't help you, obviously... the first three frequencies 
should be zero, since they are associated to translational modes of the barycenter of the 
molecule. They come out non-zero for the convergence issues you mentioned in your mail. 
You can enforce acoustic sum rules with the appropriate flags in the input (see 
Doc/INPUT_PH.txt). Anyway, I don't think you need a phonon calculation to compute the 
energy associated to the translational motion. You should pay attention only at the other 
frequencies.


> In addition, I intend to calculate the zero point energy of system by 
> PHNONS code, but it does not give the zero point energy and other 
> thermodynamics properties in output file. How could I do? I need your 
> help urgently.

No comment...


Cheers,

GS

>  
> Thanks in advance
> Best regards
> Lihui
> 
> --
> ======================================
> Lihui Ou
> PH.D Candidate in Electrochemistry                     
> College of Chemistry and Molecular Science           
> Wuhan University,/430072/,Hubei Province,China 
> /E-mail:oulihui666 at 126.com <mailto:oulihui666 at 126.com>/
> ======================================
> 
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| Gabriele Sclauzero, PhD Student                  |
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