[Pw_forum] phonon frequencies with pwscf and cpmd

[Massimiliano Bonomi]

Just some explanations.
I did actually the calculation with the same parameters ( cutoff_wfc=30, 
cutoff_rho=120,
kpoints_mesh=6 6 6): the phonon frequencies of the two codes are more or 
less the same of
the file I have attached. The negative frequencies of pwscf remain 
unchanged after using dynmat.x.
But this is not the point. My real question is if everyone can explain 
me why this two codes
give such different results in this particular calculation, while the 
other analysis on this system
(energies of the occupied and unoccupied states, for example) are 
exactly the same.
Can it be due to the use of two different pseudopotentials?

Thanks for your help. Best regards,
Massimiliano Bonomi

>On Tuesday 23 March 2004 10:57, Massimiliano Bonomi wrote:
>
>>/ I'm using both pwscf and cpmd codes with the same parameters
>/>/ (cell dimension, cutoff for the wavefunctions, number of kpoints...)
>/
>>/ WAVEFUNCTION CUTOFF(RYDBERG):                           15.00000
>/>/ DENSITY CUTOFF(RYDBERG):          (DUAL= 4.00)          60.00000
>/>/ NUMBER OF SPECIAL K POINTS (IN CARTESIAN COORDINATES):       275
>/ 
>>/     kinetic-energy cut-off    =      20.0000  Ry
>/>/     charge density cut-off    =     100.0000  Ry
>/>/     number of k points=  258  [..]
>/
>doesn't look the same to me. There is no reason to use a non-default
>cutoff for the charge density, by the way.
>
>>/ pwscf's phonon frequencies are not all positive
>/
>see http://www.pwscf.org/guide/2.0/html-node/node34.html , under 
>"ph.x does not yield acoustic modes with omega=0 at q=0"
>
>>/ For comparison I've calculated phonon frequencies with pwscf using 
>/>/ car-parrinello finite differences method moving manually each atom.
>/>/ I found negative frequencies which are in agreement with the ones
>/>/ I found with perturbation method.
>/
>which demonstrates that the phonon code works
>