# Introduction

Electronic energy transfer (EET) plays an important role in natural systems, most prominently as an essential step in photosynthesis(Scholes 2003). Related to EETs are excitonic interactions, i.e. the interaction between excited states on different chromophores. Of particular recent interest are excitonic resonance states that arise when an excitation is a combination of locally excited states on different chromophores giving rise to a delocalized excited state.

The quantity of interest in EETs are the electronic couplings that determines how much the different chromophores interact. The larger this coupling is, the more delocalized the excited state can be. In order to understand the photophysics in large multichromophoric systems, the electronic coupling are essential to evaluate and understand.

There are essentially two ways to calculate the couplings:

• taking into accout Coulomb, exchange and overlap terms.
• transformation to diabatic state.

In this work, we use the first approach.