Molecular Orbital Theory theory for polyatomics
The simple approach we used for diatomic molecules is more difficult for polyatomic molecules. Even for a simple triatomic like H2O drawing the interactions between orbitals is challenging because there is no single geometric axis to guide our evaluation of orbital overlaps.
Symmetry and group theory can be valuable tools for addressing this difficulty in polyatomic systems. Orbitals are treated as objects that can be labeled by their irreducible representations. This can be done within either the VBT or LCAO-MO approaches to covalent bonding. We then use group theoretical techniques to understand the properties of the molecular orbitals.