Atomic units are a system of measurement used primarily in atomic and molecular physics. They simplify calculations by setting fundamental constants, such as the mass of an electron and the charge of an electron, to one. This allows scientists to express physical quantities in a more manageable way, making it easier to understand the behavior of atoms and molecules. In atomic units, the unit of length is the Bohr radius, the unit of mass is the mass of an electron, and the unit of energy is the Hartree. This system is particularly useful in quantum mechanics, where it helps streamline complex equations and enhances clarity in theoretical work.