[Pw_forum] RE: on the finite electric field method

[umari at democritos.it]

Dear Liu,

For calculating Raman spectra of large model structure I suggest the method
described in :

P. Umari, A. Pasquarello “Infrared and Raman spectra of disordered 
materials from first principles”, DIAMOND AND RELATED  MATERIALS 14, 
1255 (2005).

The idea is that the Raman tensor is calculated by taking finite differences
of atomic forces in the presence of an electric field

For the calculation of the whole tensor, two different electric fields are
required.
In the code cp.x (see the documentation file: INPUT_CP ) the 
possibility of up two different electric fields is implemented.
Please consider that:
1)atomic forces and electric fields are implemented for the US 
pseudopotentials
only in the cp code 2)for parallel calculations onlt the option gdir=3 
is supported

Both limitations will be overcome with a new version in about one-month time.


Best wishes,
PaoloU



> 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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