  Organic electronics is a rapidly developing technology. Typically, the
molecules involved in organic electronics are made up of hundreds of atoms,
prohibiting a theoretical description by wavefunction-based ab-initio methods.
Density-functional and Green's function type of methods scale less steeply with
the number of atoms. Therefore, they provide a suitable framework for the
theory of such large systems.
  In this contribution, we describe an implementation, for molecules, of
Hedin's GW approximation. The latter is the lowest order solution of a set of
coupled integral equations for electronic Green's and vertex functions that was
found by Lars Hedin half a century ago.
  Our implementation of Hedin's GW approximation has two distinctive features:
i) it uses sets of localized functions to describe the spatial dependence of
correlation functions, and ii) it uses spectral functions to treat their
frequency dependence. Using these features, we were able to achieve a favorable
computational complexity of this approximation. In our implementation, the
number of operations grows as N^3 with the number of atoms N.
