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The Rydberg (or Rydberg unit of energy) is the ionization energy of hydrogen in its ground state, which is around 13.6 eV, currently calculated to be 13.605693122994 eV and 2.1798723611035 × 10-18 joules. The Rydberg constant is this value divided by hc, the Planck constant (h) times the speed of light in a vacuum (c). The Hartree unit of energy is exactly twice the Rydberg unit.