[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,
I understand what you're writing that DFPT allows you to simulate a monochromatic perturbation of any periodicity with a unit cell (learned from the tutorial), but for frozen phonon, I have no idea yet and need to study more about its basics.
For short, does  it mean for DFPT→1 unit cell is ok, and                                      frozen phonon→need supercell?

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) ? 

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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