This thesis presents new theoretical developments to the Multi-Configurational short-range Density Functional Theory (MC-srDFT) model. The focus of these developments are centered around extending the models capabilities to better describe the intrigue electronic structures of transition metal containing complexes. The extension of the model’s capabilities has been achieved both in terms of molecular properties and through the development of a short-range exchange-correlation functional. The Multi Configurational short-range Density Functional Theory (MC-srDFT) model is a hybrid model combining a long-range wave function, which in this work is primarily represented by a Complete Active Space (CAS) wave function. This wave function effectively encompasses the long-range static correlation effects, and when combined with a short-range KS-DFT exchange-correlation functional, it also effectively captures short-range dynamic correlation. The range-separation is achieved through the error function, thereby removing any double-counting of electron correlation. By substituting spin-polarization with an on-top pair density, the model eliminates the incorrect MS -dependency typically inherited from the single-determinant KS-DFT, leading to the development of the new Multi-Configurational short-range on-top pair-density Density Functional Theory (MC-srPDFT) model. Implementation of GIAO within the MC-srDFT model enables the calculation of gauge-independent Nuclear Magnetic Resonance (NMR) shielding constant for transition metal systems, outperforming regular DFT and CASSCF. Singlet linear response for an open-shell wave function is also implemented enabling the calculation of excitation energies and oscillator strengths with MC-srDFT. Furthermore, the Self-Interaction Error (SIE) is examined for both MC-srDFT, and MC-srPDFT, demonstrating that both model are free from SIE contrary to other on-top pair density approaches.