An Etymological Dictionary of Astronomy and Astrophysics

A unit of energy used in atomic and molecular physics; symbol Ha or E_h. It is defined as: 1 Ha = m_ee⁴/(4ε₀²ħ),
where m_e is the mass of electron, e its charge, ε₀ the → permittivity of vacuum, and ħ → reduced Planck’s constant. Its value is 2 → rydbergs, or 4.3597 x 10^-18 → joule, or 27.213 → electron-volts.

See also: Named for the British physicist and mathematician Douglas R. Hartree (1897-1958).