  In large photosynthetic chromophore-protein complexes not all chromophores
are coupled strongly, and thus the situation is well described by formation of
delocalized states in certain domains of strongly coupled chromophores. In
order to describe excitation energy transfer among different domains without
performing extensive numerical calculations,one of the most popular techniques
is a generalization of Forster theory to multichromophoric aggregates
(generalized Forster theory) proposed by Sumi [J.Phys.Chem.B,103,252(1999)] and
Scholes and Fleming [J.Phys.Chem.B 104,1854(2000)]. The aim of this paper is
twofold. In the first place, by means of analytic continuation and a time
convolutionless quantum master equation approach, a theory of emission
lineshape of multichromophoric systems or molecular aggregates is proposed. In
the second place,a comprehensive framework that allows for a clear,compact and
effective study of the multichromophoric approach in the full general version
proposed by Jang, Newton and Silbey [Phys. Rev. Lett.,92,218301,(2004)] is
developed. We apply the present theory to simple paradigmatic systems and we
show: the effectiveness of time-convolutionless techniques in deriving
lineshape operators; how the multichromophoric approach can give significant
improvements in the determination of energy transfer rates in particular when
the systems under study are not the purely Forster regime. The presented scheme
allows for an effective implementation of the multichromophoric Forster
approach which may be of use for simulating energy transfer dynamics in large
photosynthetic aggregates, for which massive computational resources are
usually required. Furthermore,our method allows for a systematic comparison of
multichromophoric Foster and generalized Forster theories and for a clear
understanding of their respective limits of validity.
