This thesis is concerned with two topics from the field of computational electronic structure theory. In the first part we will describe our contributions to the ab initio density matrix renormalization group (DMRG) methodology in quantum chemistry, in particular our quadratically scaling local algorithm. We will present applications aimed towards new insights into the physics of conjugated π-electron systems as found, e.g., in organic electronic materials. The second part of this thesis covers our computational study of 3d-M(smif)2 complexes synthesized and characterized in the Wolczanski Group. These compounds exhibit unusual electronic structure phenomena which we address from a theoretical perspective.