[DFTB-Plus-User] external electric field for periodic systems

[Visikovskiy Anton]

Dear all,

I study graphene and graphene nanoribbon related materials and would 
like to use DFTB+ for IR and Raman spectra simulation via Fourier 
transform of time-correlation functions (FT-TCF), which basically 
involves finite temperature MD simulation with accumulation of data on 
such parameters as dipole moment and polarizability. Well, it is pretty 
much straight forward with the former one, as I can just extract 
Mulliken charges from program output files at each time step and 
calculate the total dipole. It is a bit trickier with polarizability. 
According to this paper - S. Kaminski et al., J. Chem. Theory Comput 6 
(2010) 1240, it is possible to extract components of polarizability 
tensor by calculating system's total energy second derivatives with 
respect to electric filed components by finite difference method. So, 
basically I have to calculate the energies of the system with a given 
atomic structure applying some external electric field in different 
direction. As far as I understood from the manual of DFTB+ 1.2 the 
external field cannot be applied for periodic systems in the direction 
of periodicity due to saw-tooth shape of the potential. So, is there any 
workaround this problem using DFTB+ code, so I could calculate energy 
response of the system to arbitrary oriented external field for periodic 
systems such as graphene sheet?

Best regards,
Anton Visikovskiy.