In 1928, Paul Dirac proposed his fundamental covariant equation of motion that governs the relativistic dynamics of a single electron in a classical electromagnetic field. This equation became the basis not only of more fundamental theories such as quantum electrodynamics but also of endeavors to solve many-electron problems in chemistry, molecular physics, and materials science -even if they originally set out from Schrödinger's (nonrelativistic) formulation of quantum mechanics.