Basic concepts

The electronic structure of molecules is described, starting from qualitative Molecular Orbital (MO) theory. After the case of simple atoms and molecules, one treats molecular solids and develops the relation between Molecular Orbital theory and band theory. In both cases, one shows that the electronic structure can influence the geometrical structure, through Jahn–Teller effects or Peierls distortion. The effect of interelectronic repulsion, the central problem of Quantum Chemistry, is put in perspective by a synthetic presentation of different approaches: Hartree–Fock Self-Consistent Field with treatment of electron correlation, Valence Bond models, and finally Density Functional Theory methods (DFT). The last section is devoted to quantum tunnelling and its dynamical aspects.