A Hartree (symbol E_h) is the atomic unit of energy and is named after physicist Douglas Hartree.

The Hartree energy is equal to the absolute value of the electric potential energy of the hydrogen atom in its ground state. This is also almost exactly twice the absolute value of the binding energy (or ionization energy) of the electron in the ground state hydrogen atom, |W₁| (it is not exactly equal to twice the ionization energy, because of the finite mass of the proton; see reduced mass).

$E\_h\; =\; \{\backslash hbar^2\; \backslash over\; \{m\_e\; a^2\_0\}\}\; =\; m\_ec^2\backslash alpha^2$

= 4.359 744 17(75) J = 27.211 3845(23) eV = 2 Ry = 627.509 391 kcal/mol

where:

$\backslash hbar\; \backslash$ is the reduced Planck constant,

$m\_e\; \backslash$ is the electron rest mass,

$a\_0\; \backslash$ is the Bohr radius,

$c\; \backslash$ is the speed of light in a vacuum, and

$\backslash alpha\; \backslash$ is the fine structure constant.

Units of energy | Physical constants

Hartree-Energie | Energia di Hartree | ハートリー原子単位 | Hartreejeva energija