The real Coulomb interaction between electrons is given by
and the full hamiltonian is
Instead, Hartree hamiltonian is
Taking the difference between
We can consider
as a perturbation with respect to
, representing the difference between the actual coulomb interaction between electrons and the avarage electron repulsion.
The eigenstates of
are labeled with the good quantum numbers
and are degenerate in
since energies depend only on
In order to deal with simpler calculations we'd like to find a basis that makes the perturbation
diagonal together with
If is the most relevant perturbation  and thus we do not have to deal with other terms in the hamiltonian, we can proceed as follows.
Since does not contain spin-orbit energy terms it commutes not only with but also with and . Furthermore, so that is still a good quantum number together with .
The right way of labeling states is thus:
Using Clebsch-Gordan coefficients we can easily change basis:
The degeneracy of energy levels is partially removed by :
States with different
are no longer degenerate.
This is the reason why when we take into account non-spherical corrections we label energy levels no longer with
but rather with atomic terms in LS scheme
where the capital letter
has to be replaced with
, which represents 15 degenerate states, is now split in
and each of these terms with a different capital letter
has different energy.
After having taken into account non-spherical corrections, we can consider other less relevant perturbations such has spin-orbit coupling: as we'll see in the next chapter the effect of this perturbation is to split the terms according to their , since
This leads to a further removal of degeneracy, since states such as
no longer have the same energy.
The physical fact partially hidden by all the maths is that spins interact giving a total spin and so do the orbital angular momenta , which give rise to a total angular momentum . Then and couple into a (LS coupling).
In order to find the term with the lowest energy the following empirical rules hold:
- The term with the maximum spin has the lowest energy
- If there still are more terms, choose the one with the highest angular momentum
- if the shell is less than half-full the lowest energy term is the one with the lowest value of J, otherwise the one with the highest value of J
- ↑ in particular, if spin-orbit effects are small
- ↑ For a detailed discussion about how to find atomic terms read LS scheme