BerkeleyGW is a many-body perturbation theory code for quasiparticle calculations in the GW approximation and optical calculations with the Bethe-Salpeter equation (BSE). It is freely available from http://www.berkeleygw.org. Octopus can produce the mean-field input for a BerkeleyGW calculation by setting Output = berkeleygw in the input file, if Octopus has been linked with the BerkeleyGW I/O library.
The code and its formalism are described in this paper:
Jack Deslippe, Georgy Samsonidze, David A. Strubbe, Manish Jain, Marvin L. Cohen, and Steven G. Louie, "BerkeleyGW: A Massively Parallel Computer Package for the Calculation of the Quasiparticle and Optical Properties of Materials and Nanostructures," accepted to Comput. Phys. Commun. (2011) (http://arxiv.org/abs/1111.4429)