Hartree energy (Ha) is named after British physicist Douglas Hartree. It is the measure of atomic unit of energy in atomic physics and computational chemistry. It is an energy roughly equal to the electric potential energy of a hydrogen atom in the ground state or twice its ionization energy. It is expressed as 2R∞hc, where ∞ is Rydberg constant, h is the Planck constant and c is speed of light. Its recommended value is 4.359 744 17(75) × 10-18 J.