[Pw_forum] on the finite electric field method

[li niu]

Dear all ,
  
   I want to investigate Raman spectra in the case of large disordered system from first principles. I saw the exmple30 and 31, but they only gave the dielectric constant and Born effective charge by Moden polarizability theory.
     How can I obtain Raman coupling tensor by numerically calculating second derivatives of the atomic forces with respect to the electric field? 
   [in RAMAN SPECTRA OF DISORDERED OXIDES FROM FIRST   PRINCIPLES  written by PAOLO UMARI]? 
     
  I did not find those from the forum.
 
Thank you in advance!
 
Niu Li



       
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