[Pw_forum] phonon frequencies with pwscf and cpmd

Paolo Giannozzi giannozz at nest.sns.it
Tue Mar 23 15:31:03 CET 2004

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
>     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

Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy

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