Which spin state does the ion take?

You will write functions that count pairs, work out the total energy of an octahedral arrangement, and choose the spin state of an ion.

The idea. The total energy is the CFSE plus the pairing energy for each pair beyond those of the free ion. The ion takes the arrangement with the lower total.

Your tasks.

1. Complete extra_pairs(d, unpaired) so that it returns the pairs beyond the free ion. The ion has $(d - \text{unpaired})/2$ pairs and the free ion has max(d - 5, 0).
2. Complete total_energy(t2g, d, delta, pairing) so that it returns the CFSE plus the pairing cost, in kJ/mol.
3. Complete spin_state(d, delta, pairing) so that it returns "high" or "low", whichever has the lower total. A tie is "high".