[Pw_forum] on finite electric methods to calulate vibrational properties

li niu niuli1978 at yahoo.com.cn
Tue May 1 04:07:26 CEST 2007

Dear Stefano Baroni ,
  I'm sorry for my spelling error. 
   I want to investigate Raman spectra in the case of large   disordered system from first principles and use the finite electric field methods.
The dielectric polarizability tensor can be calculated by taking dinite differences of  atomic forces.
There are several questions to ask you.
  1. We obtained the diagonal terms, by considering values for the electric field of
-2h,-h, 0, h and 2h in the cp.x code[tefield=.true.  
                                                         tcg = .true.,
                                                     epol = 1 or 2 or 3
                                                    efield = -2h,-h, 0, h or 2h],
and by using the five-point formular . Is it right?
  2. For the calculation of the off-diagonal terms, two different electric fields 
are required. This need apply electric fields of intensity -2h/1.414,-h/1.414, 
0, h/1.414 and 2h/1.414 along the three different couples of cartesian directions, and apply eauation.
But how can I apply simultaneously finite fields along two different Cartesian directions
in cp.x code?[ESSPRESSO-3.2]
3. In this way, we could obtain all the dielectric polarizability tensor. Then the
Raman susceptibilities associated to the normal mode n can be obtianed according to the equation. 
How to implement it ? By a small code?
  I'm counting on the new version.
  Niu Li

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