"The hartree (symbol: Eh or Ha), also known as the Hartree energy, is the atomic unit of energy, named after the British physicist Douglas Hartree. It is defined as 2R\u221Ehc, where R\u221E is the Rydberg constant, h is the Planck constant and c is the speed of light. The 2010 CODATA recommended value is Eh = 4.359\u2009744\u2009650(54)\u00D710\u221218 J = 27.211\u2009386\u200902(17) eV."@en .