Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) are the primary tools for general applications involving ground and excited electronic states, respectively. However, the single-reference formulation of DFT imposes significant limitations on systems with multi-configurational characteristics, such as open-shell singlet cases including diradicals and bond-breaking events. TDDFT also has well-known inherent failures in describing the energy of long-range charge transfer excitations, excited states with substantial double excitation character, excited states of molecules undergoing bond breaking, and the topology of conical intersections (CIs). Perhaps, the correct description of conical intersections (CIs) is one of the most challenging requirements of electronic structure theory, as it should produce non-vanishing nonadiabatic coupling between the intersecting states. Surprisingly, this requirement for the S 1 /S 0 CIs is violated by most single-reference theories including single-state multi-reference computational methods such as single-state (SS)-CASPT2. Multi-State multi-reference computational methods are capable of producing the correct topology of CIs, however at the expense of very high cost of computations. A solution to this problem is to develop a unified and efficient quantum mechanical theory that can accurately describe both ground and excited electronic states on an equal footing.