Due to the unfavorable scaling of quantum chemical methods one usually has to compromise between accuracy and computational effort with growing system size. Finding approximations that overcome this constraint sparked the interest of researchers which eventually led to the development of so-called multiscale methods. This thesis pertains to a multiscale method called Frozen-Density Embedding theory (FDET), in which the system is described by means of two independent quantum mechanical descriptors, the wavefunction of the embedded species and the charge density of the environment. This work examined the effect of the non-linearity of FDET equations. The second topic of this thesis was the development and implementation of FDET-based methods for ground and excited states. For this purpose the fdeman module, which manages all steps of an FDET calculation, was implemented into the quantum chemistry software package Q-Chem.