The weird electron configuration of Niobium was well explained on this post:
How can one explain Niobium’s weird electronic configuration?
It was pointed out that electrons fill orbitals with higher exchange energy and lower repulsion energy, only if the sum of the positive exchange energy and negative repulsion energy overcomes the energy gap between orbitals.
The only problem is that I do not know how to quantify the energy gap.
How can I calculate the energy gap between orbitals?