[Pw_forum] help: the phonon calculation

[hailin yu]

Dear all:
     I have some quesitons want to know. Can you tell me ? Thanks a lot!
1).Recently, I had calculated Ag3N which has the anti-ReO3 strucuture (i.e, 
has the same structure with Cu3N,PRB 53,12648) by using CASTEP and found 
it's mechnical stabel  ( I find the elastic siffness coefficients of Ag3N 
satisfy the generalized elastic stability criteria). But when I calculated 
the phonon dispersion of Ag3N using PWSCF, I found there was a soft mode 
along G-X direcition (i.e the phonon frequency is negative). For a negative 
frequency appear, it means that the structure is not stable? If so, the 
results of CASTEP and PWSCF is confict? This quesiton is still confuzed me 
and I am eager to receive your reply. Thanks!

2). For f-electronic materials (such as Pt), for their lattice dynamics 
properties, it is reasonable to calculate using PWSCF ?

Thanks!

     yuhailin

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