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The Rydberg (or Rydberg unit of energy) is the ionization energy of hydrogen in its ground state, this amount of energy being roughly 13.6 eV, more precise current determinations being 13.605693122990 eV and 2.1798723611030 × 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.