[Pw_forum] the difference between the linear response method and finite differences of the atomic forces

[li niu]

dear all,
 I found there are two methods to obtain the vibarational modes and frequencies by diagonalizing the dymamical matrix. One is the linear response method and the other other is finite differences of the atomic forces by displacing each atom and computing the resulting forces.
 My question is that what's the difference between these two methods? And what's the advantage of respective method?
 
 Thank you in advance!
 
 Best wishes,
 
  Niu Li
 
  Harbin Institue of Technology 
                                                                     
  China
 		
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