The Heitler London Method is a quantum mechanical theory used to explain the bonding between atoms in a molecule. It was developed by Walter Heitler and Fritz London in the 1920s.
S and T refer to the Symmetry and Translation operators, respectively, used to describe the behavior of electrons in a molecule.
The Heitler London Method uses the principles of quantum mechanics to describe how atoms share electrons in order to form chemical bonds. It explains bonding as a result of the overlap of atomic orbitals.
The Heitler London Method is limited in its ability to accurately predict the bonding behavior of molecules with multiple bonds or complex molecular structures. It also does not account for the repulsion between electrons in the same orbital.
Although it has its limitations, the Heitler London Method is still used as a foundation for other bonding theories in modern chemistry. It is also used in computational chemistry to calculate molecular properties and predict chemical reactions.