Heitler-Watson theory

The Heitler-Watson theory explains how pairs of electrons in molecules behave, especially in covalent bonds. It describes how two electrons with opposite spins can be shared between two atoms, forming a stable bond. This theory introduces the idea of wavefunctions—mathematical descriptions of electrons' behaviors—and uses them to show that bonding electrons are not bound to just one atom but are delocalized over both. Essentially, it provides a quantum mechanical foundation for understanding how atoms stick together by sharing electrons, leading to the stable structures of molecules.