[gpaw-users] Dielectric Function

[Jun Yan]

Hi, Torsten,

    Use  df1, df2 = df.get_dielectric_function()
    where df1 and df2 are the dielectric function calculated with and 
without local field effects using RPA. The ALDA kernel is not 
implemented for optical excitations since it doesnt improve results.
    usually one use np.real(df2) and np.imag(df2) to get the real and 
imaginary part of dielectric function.

    The df.get_dielectric_function() works in two ways:
    1) start a new calculation if this object is void. but then you have 
to define a df object by (in the following example I write the least 
parameters you have to input. Please also provide a gpw file that 
contains the pseudo-wavefunction. you can get it by using the 'all' 
option in the write method : calc.write('filename.gpw', 'all')):
        from gpaw.response.df import DF
        df = DF('filename.gpw', q = np.array([0.0001,0,0], 
w=np.linspace(0,20,201)), optical_limit=True )
        df1, df2 = df.get_dielectric_function()
    2 ) output the values if they are available, but you have to save a 
previous calculation using
       df.write('filename.traj')
      and using
      df.read('filename.traj')
      df1, df2 = df.get_dielectric_function()
      you can also get these values.

Best,
Jun


Torsten Hahn wrote:
> Hi Jun,
>
> Am 11.01.2011 um 09:50 schrieb Jun Yan:
>
>   
>> Hi, Torsten,
>>
>>  Do you mean that you want to get the diagonal elements (eps_x,y,z) of a 9 component dielectric tensor ?   If so, you can find tutorial and manual at https://wiki.fysik.dtu.dk/gpaw/tutorials/dielectric_response/dielectric_response.html#df-tutorial
>> https://wiki.fysik.dtu.dk/gpaw/documentation/tddft/dielectric_response.html#df-theory
>>  The current version can't support calculating the off-diagonal elements (fx: eps_xy, eps_xz) of the dielectric tensor   .
>>
>>  The dielectric function, which is both wave-vector (q) and frequency (w) dependent, is calculated in a different way for excitations with photons (q-->0) and electrons (finite q).  If you are interest in optical excitations, you can follow the example 1 in the above tutorial.
>>
>>     
>
> sorry for the unclear expression. Yes i need the diagonal elements of the dielectric tensor. My question is rather technical: How do i access the values? The functions get_dielectric_function() etc. seems not to give access to the values ...
>
> Thx,
> Torsten.
>