The Hartree energy (symbol E_h) is a physical constant used as atomic unit of energy, named after physicist Douglas Hartree.

It has a value of twice the absolute value of binding energy of the electron in the ground state of the hydrogen atom |W₁| or the ionization energy.

= 4.359 743 81(34) × 10^-18 J = 27.214 4 eV = 2 Ry

where:

is the Planck's constant,

m_e is the electron rest mass

a₀ is the Bohr radius