[Pw_forum] finite dielectric methods

li niu niuli1978 at yahoo.com.cn
Mon Apr 30 00:31:48 CEST 2007


Dear Paolo U,
   
  Thanks for your reply.I'm very interested for your finite electric field method.
The dielectric polarizability tensor can be calculated by taking dinite differences of atomic forces.
I have 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 [like example 30
                                                       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 mixedterms, 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 equations.
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 equations. 
How to implement it ? By a small code?
   
  Best!
   
  I'm counting on the new version.
   
  Niu Li

       
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