From Schrödinger to Einstein and Dirac: Relativistic Effects

The implications of Einstein’s special relativity in chemistry are discussed. It is shown that relativistic effects on the electronic structure of an atom or molecule scales in leading order as Z2, where Z is the charge number of the heaviest nucleus in the system. Well-known heavy atom effects in chemistry are discussed: The color of gold, the liquid state of mercury, the inert pair effect of heavy p-block elements, and more. Spin-orbit coupling (SOC) is also a relativistic effect and plays a big role in spectroscopy and chemistry. The Dirac equation (DE) replaces the electronic Schrodinger equation in relativistic quantum chemistry. The Dirac wavefunctions have 4 components. It is shown how an ‘exact 2-component’ (X2C) Hamiltonian can be constructed. X2C based all-electron calculations are becoming increasingly popular in quantum chemical applications. Molecular properties may undergo a picture-change effect when going from a 4-component to a 2-component framework.