Because the multi-d -occupancy character can be measured, the proposed definition of the double d -shell effect is more quantitative than the usual definition, although it is still somewhat arbitrary as it depends on how the d -orbitals are localized. Here we will use an indirect, but straightforward, procedure to measure the d -occupancy mixing, based on the metal 3d population (d pop ). Starting from Equation ( ) for the division of the wave function in the contribution of two occupancies, the 3d population may be written as