orbitals of compatible symmetry can combine, or mix, even when they have different energies
when sets of orbitals mix, it has the effect of decreasing the energy of the lower-energy set and increasing the energy of the higher-energy set
there are two ways to explain mixing: 1. starting from the simplified MO diagram and mixing molecular orbitals of like symmetry, and 2. starting from and mixing atomic orbitals of compatible symmetry
