Molecular Orbitals in the HF Theory

The passage from atomic orbitals (AOs) to Molecular Orbitals (MOs) is not so simple as it could appear at first glance. The best sentence apt to resume the task is, in my opinion, that one expressed long ago by Roothan: “The basic concept of the Molecular Orbital method is to find approximate electronic wave functions for a molecule by assigning to each electron a one-electron wave function which in general extends over the whole molecule”.