First principle (ab initio) methods provide significant insight about the electronic structure (ES) and physical properties of structures of interest in physics, chemistry, biology and materials science. Some of these methods are useful for understanding the electronic structure of active sites in metalloproteins and for interpreting experiments that probe their ground or excited states. In particular, Kohn-Sham density functional theory (KS-DFT) is a powerful method for studies of electronic structure of large (e.g., 60-200 atoms) transition metal-containing complexes. Density functional theory takes into account electronic exchange and correlation effects which are important for the description of spin-polarized open-shell transition metal clusters such as polynuclear active sites in proteins. Indeed, density functional theory in conjunction with high performance supercomputers is becoming a powerful tool for studies of biomolecular electronic structure and biomolecular magnetism.