# [Pw_forum] help: the phonon calculation

Stefano Baroni baroni at sissa.it
Mon Aug 28 13:44:57 CEST 2006

```On Aug 28, 2006, at 7:43 AM, lan haiping wrote:

>
>
> On 8/28/06, hailin yu <yuhailin_79 at msn.com> wrote:
> 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
>
>
>  you should give your input setting of calculation
>

This question deserves a more general answer that does not depend on
any specific input of any specific calculation. The answer is kind of
trivial, once one fully understands the question, but I am afraid
that Halin Yu has some problems with the question itself. Suppose you
are at the saddle between two valleys in a mountain landscape. Forces
acting on you are zero. Is the equilibrium stable or unstable? Well,
it depends on the direction: if you keep climbing up to the top of
one mountain nearby, you will say that the equilibrium is stable. If
you roll downhill, you would say instead that the equilibrium is
unstable. Suppose now that your map only indicates the path climbing
up. You would positively conclude that the euilibrium is stable. This
is the conclusion that you drew from your CASTEP calculation that
state that the equilibrium is stable with respect to distortions
which change the shape of the unit cell (elastic stability criteria).
These criteria do not say anything about the stability with respect
to a phonon distortion which, by definition, does not change the
shape of the unit cell.

> 2). For f-electronic materials (such as Pt), for their lattice
> dynamics
> properties, it is reasonable to calculate using PWSCF ?
>
> as far as i know, there is no problem for pwscf to calculate
> metal's lattice dynamics ,
> since it is  Pseudopotential -plane wave method..

as long as you do things properly ...

Stefano B.

>
>
> regards,
>
> hai-ping
>
> Thanks!
>
>     yuhailin
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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