ABSTRACT
In this chapter, we explain the theory used to treat electronic
excitations in multichromophoric systems like biological light-
harvesting complexes and DNA as well as vibrations in peptides.
Although these systems appear very different, they can be treated on
a common theoretical footing. Twomain issues will appear through-
out this chapter. First, the systems under study are composed
of multiple chromophores interacting with light. These building
blocks are, however, not independent, but coupling between them
leads to collective behavior observable in the optical response. The
second important ingredient of the theory is interaction of the
chromophores with a complex environment, which tends to destroy
the cooperativity.
