[QE-users] In Phonon Calculation for Raman Spectra, How to decide to use how many unit cells?

[ludwigboltzmann.sc10 at nycu.edu.tw]

 Dear L,
Thanks for your clear explanation, I understand it now.

Best Regards, 
HY Lu


    在 2023年11月15日 星期三 下午06:06:21 [GMT+8]， Lorenzo Paulatto<lorenzo.paulatto at cnrs.fr> 寫道：  
 
  
 
     Sorry but for 1 more naive question,  if I use 2 unit cell instead of 1 in DFPT, would it be expected to have difference between the two cases ( only at Gamma point) ? 
      
 
yes, it is different. Because you would get twice as many phonons, but half of them would be periodic inside the supercell (and correspond to Gamma) the other would not be (and correspond to a zone-border q-point).
 
hth
 
 

 
 
     
  
  Best Regards,  
  HY Lu
  
  
  
  
  
      在 2023年11月15日 星期三 下午03:56:04 [GMT+8]， Lorenzo Paulatto<lorenzo.paulatto at cnrs.fr> 寫道：  
  
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 On 15/11/2023 08:45, ludwigboltzmann.sc10 at nycu.edu.tw wrote:
  
 
     Hope this will give you an idea why such symmetry reduced supercell calculation is being carried out.  

 
 
The idea is "symmetry reduced".
 
I.e. DFPT allows you to simulate a monochromatic perturbation of any periodicity with a unit cell, while frozen phonon can only simulate perturbation which have the periodicity of the cell you are using. But to break symmetry, you need a supercell.
 
 
kind regards
  
L
 
     
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 Dr. Lorenzo Paulatto
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