[Pw_forum] Negative frequencies in phonon calculation

[Stefano de Gironcoli]

Is the system optimized also for what concerns the chian lenght ?
In an infinite system vanishing forces is not the only condition for 
equilibrium, also stress (chain tension in your case) should vanish
 
Stefano de Girocoli

Alberto Milani wrote:

>Dear Jose,
>Thank for your help.
>I know well the problems about saddle points and negative frequencies because 
>I usually calculate normal modes of molecules where this problem occurs many 
>times.
>Here the problem is that I am studying an infinite linear chain and so I found 
>strange that some kind of saddle point could be the source of the problem for 
>such a system. Many other ab-initio calculation on polyyne finite chain have 
>shown that these systems are indeed stable as linear chains (no negative 
>frequencies are found). So I would exclude any kind of transversal 
>deformartion from linearity.
>From this point of view, there is no possibility for any saddle point and the 
>relaxed structure that I have obtained should be considered converged in a 
>well defined minimum. In fact, no negative frequencies are found for the 
>longitudinal phonons.
>So I think that the problem here is not the usual saddle instability. I wonder 
>if the code fails in some other way in stabilizing the linear chain (e.g. 
>choice of the functional, pseudopotential, ...).
>Thank you again for your attention.
>Yours,
>A. M.
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