One of the greater challenges of modern quantum chemistry is to meet the growing demands of a detailed description of macro molecules, such as proteins and RNA/DNA. The size of such systems make it an almost impossible task to, from first principle calculations, obtain any detailed information regarding electronic structure. This thesis is focused on development of new computational tools for investigation of electronic transitions within large molecular systems. Central to this is the polarizable embedding (PE) model. For many of these systems electronic transitions are localized to one or a few active sites which in turn will be described by accurate quantum chemical methods. The remaining part of the system, defining the environment, is then treated at a lower level using a classical electrostatic approach. The environment is further incorporated into the Hamiltonian describing the quantum region in an effective manner.

The work presented in this thesis covers extensions to the PE model both in the quantum region and in the environmental description. The former, starts by combining the PE model with a damped linear response formalism. This coupling enables the study of electronic processes near resonance and in any frequency range desirable thus covering all from core hole transitions to valence transitions. Another important aspect of this work is the coupling of PE to a polarizable continuum model which allows for inclusion of bulk solvent  effects in an effective manner. In addition to these extensions we have further considered alternative ways of acquiring parameters used in the construction of the embedding potential. This is essential for accurate embedding calculations as the embedding potential enters directly in the wave function optimization.