# Fourth principle of Molecular Orbitals

The fourth principle of Molecular Orbitals state that:

Molecular orbitals are best formed when composed of Atomic orbitals of like energies.


I'm not sure about how I should interpret this; which is the correct?

• It means that the molecular orbitals that I need to consider are the ones formed by the same orbitals on each atom (like a pair of $2p$, two $3s$, etc).

• It means that I need to evaluate the actual energy on each orbital using: $$E = \frac{-13.6 z^2}{n^2}$$ And then compare the energies of every orbital, to match each orbital on Atom 1 to the closest one on Atom 2? Meaning that I could possibly have a $2s$ pairing with a $3p$, depending on both atoms? (Note that the formula gives different results for each Atomic Orbital depending on the atom's $z$).

I known that every possible orbital combination exists, but I'm asking about the ones that are more relevant and thus need to be considered.