ABSTRACT
Heitler and London in 1927 gave the description of the chemical bond, which turned out to be a great early achievement of quantum mechanics. Prior to this time it was impossible to explain why two hydrogen atoms come together to form a stable chemical bond. It was later seen that existence of a stable chemical bond was a quantum mechanical effect. The simplest molecule forming a chemical bond is hydrogen molecule, formed due to bonding between two hydrogen atoms. We shall discuss hydrogen molecule and hydrogen molecule ion in detail since most of the ideas and techniques developed for hydrogen molecule will be applicable to more complex molecules.
