[Pw_forum] Negative Phonon Frequencies - Numerical instability or unstable structure

[Pietro Delugas]

Sorry forgot half of the answer


acustic branches should be flat along k_z direction, I would check that 
is so. If it is not so, maybe you are adding non physical interactions 
between periodic images and this can cause the numerical instabilities. 
   I don't know whether your program is able to clean spurious 
interactions due to fake periodicity.
  I hope this helps - pietro

On 05/12/2016 09:52, Pietro Delugas wrote:
>
> Then one of the 3 acustic branches ( probably the one you gave 
> problems with ) should be flat.
>
> I don't know how this phononpy works,  maybe there is a way to deal 
> directly with 2D systems, otherwise just find a way to set to 0 all 
> the interatomic force constants that  correspond to interactions 
> between atoms staying in different periodic images of the slab.
>
>
> pietro
>
>
>
> On 05/12/2016 09:33, Mohammed Ghadiyali wrote:
>>
>> Hi,
>>
>>
>> The structure is two dimensional similar to bi-layer graphene.
>>
>>
>> Regards,
>>
>>
>> Ghadiyali Mohammed Kader
>>
>>
>> Research Scholar
>>
>>
>> University of Mumbai.
>>
>>
>>
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>
>
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