[Pw_forum] lattice instability and local minimum energy postion

[Stefano Baroni]

Dear Wenmei Ming,

> Dear Prof. Paolo Giannozzi:
>
>       yes, before I calculated phonon-dispersion of CrSb, I have  
> followed your PRB paper several years ago concerning with phonon  
> dispersion of semiconductors, Si, Ge,GaAs,GaSb,AlAs,AlSb,also using  
> pw91-n-van.UPF pseudopotential ,and I found that I had to set the  
> energy cutff and charge cutff larger than 40Ry and 500Ry  
> respectively to get the correct frequencies consistent with your NC- 
> LDA results,otherwise there will many imaginary frequencies for the  
> acoustic branches . Therefore, in my CrSb case, I intentionally set  
> the energy cutoff and charge cutff to be 60Ry and 600Ry, and also  
> phonon scf-consisten threshold was set to be 10-16.
>     Experimently, the bulk zinc-blende CrSb have not been  
> synthesized and only CrSb in form of very film (about 2nm) exists  
> experimentally. Thus I want to see whether the bulk CrSb is  
> dynamically stable through phonon-dispersion calculation.
>     Do I have to increase the energy cutoff to get a reliable phonon  
> result ?

it seems to me that it would be easier for to answer (just perform a  
few convergence tests), than for any of us, given that you have the  
calculation already set up ...

still, you did not reply to Paolo's most important (IMHO) suggestion.  
Did you check your frozen-phonon procedure against a case that you  
know will not display any unexpected features? (for instance, one of  
the binary semiconductors that you mentioned above)

Regards - SB

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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