The Hartree energy is a physical constant equal to twice the binding energy of the electron in the ground state (the lowest-energy state) of the hydrogen atom . When a hydrogen atom is in this state, an amount of energy equal to 0.5 Hartree is necessary to free the electron and thereby cause the atom to become an ion .
The value of the Hartree energy is approximately 4.36 x 10 -18 joule (J), or 27.2 electronvolts (eV). The constant gets its name from the 20th-century physicist Douglas Hartree. It is sometimes used as an energy unit in theoretical physics.
Also see Table of Physical Constants .