Occupational Coupling Equation 49 has two major components: the energy condition, and the occupational coupling. The occupational coupling is similar to the previous vibronic matrix coupling element, expressed using the non-adiabaticity operator (Equation ), in which one term can be omitted. However, now instead of remaining within an internal coordinate framework, we can instead incorporate density functional theory, and with it the Cartesian coordinate system. This is similar to Equation , except we now include an internal conversion vibrational matrix B qj , which consists of the mass weighted normal modes of all transition participating modes. We can recast this as :