The hartree (symbol: E_h), also known as the Hartree energy, is the unit of energy in the atomic units system, named after the British physicist Douglas Hartree. Its CODATA recommended value is E_h = 4.3597447222060(48)×10⁻¹⁸ J‍^[1] = 27.211386245981(30) eV.^[2]

The hartree is approximately the negative electric potential energy of the electron in a hydrogen atom in its ground state and, by the virial theorem, approximately twice its ionization energy; the relationships are not exact because of the finite mass of the nucleus of the hydrogen atom and relativistic corrections.

The hartree is usually used as a unit of energy in atomic physics and computational chemistry: for experimental measurements at the atomic scale, the electronvolt (eV) or the reciprocal centimetre (cm⁻¹) are much more widely used.

${\displaystyle E_{\mathrm {h} }={\hbar ^{2} \over {m_{\mathrm {e} }a_{0}^{2}}}=m_{\mathrm {e} }\left({\frac {e^{2}}{4\pi \varepsilon _{0}\hbar }}\right)^{2}=m_{\mathrm {e} }c^{2}\alpha ^{2}={\hbar c\alpha \over {a_{0}}}}$

= 2 Ry = 2 R_∞hc

= 27.211386245981(30) eV‍^[2]

= 4.3597447222060(48)×10⁻¹⁸ J‍^[1]

= 4.3597447222060(48)×10⁻¹¹ erg

≘ 2625.4996394799(50) kJ/mol

≘ 627.5094740631(12) kcal/mol

≘ 219474.63136320(43) cm⁻¹

≘ 6579.683920502(13) THz

where:

• ħ is the reduced Planck constant,
• m_e is the electron mass,
• e is the elementary charge,
• a₀ is the Bohr radius,
• ε₀ is the electric constant,
• c is the speed of light in vacuum, and
• α is the fine-structure constant.

Effective hartree units are used in semiconductor physics where ${\displaystyle e^{2}}$ is replaced by ${\displaystyle e^{2}/\varepsilon }$ and ${\displaystyle \varepsilon }$ is the static dielectric constant. Also, the electron mass is replaced by the effective band mass ${\displaystyle m^{*}}$. The effective hartree in semiconductors becomes small enough to be measured in millielectronvolts (meV).^[3]