Orbital properties Aufbau principle

This model takes a more fundamental approach by regarding a molecule as a collection of valence electrons and positive cores. Just as the nature of atomic orbitals derives from the spherical symmetry of the atom, so will the properties of these new molecular orbitals be controlled by the interaction of the valence electrons with the multiple positive centers of these atomic cores. These new orbitals, unlike those of the hybrid model, are delocalized that is, they do not belong to any one atom but extend over the entire region of space that encompasses the bonded atoms. The available (valence) electrons then fill these orbitals from the lowest to the highest, very much as in the Aufbau principle that you learned for working out atomic electron configurations. For small mole-... 

When working with the final model potentials of equations (10) and (11) in molecular applications, we are interested in valence properties and, in fact, do not need to include core orbitals in our wavefunction any more. This is so, since core-valence interaction has been accounted for implicitly in equation (10), and the sum of energies of individual cores with fixed orbitals is a constant, anyway. However, in order to get an aufbau principle for the valence wavefunction, core orbitals > have to be shifted from energies c to energies above those of the (occupied) valence ones, and this level-shift is done by... 

There are six electrons to occupy the delocalized space orbitals. By the Aufbau principle, in the ground-state each of the three lowest-energy space orbitals is occupied by two electrons with opposite spins, as depicted by arrows in Figure 21.8. Although these orbitals do not have simple angular momentum properties, we refer to them as pi orbitals. 

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