hartree

(atomic physics) A unit of energy used in studies of atomic spectra and structure, equal to 2R_∞hc or α²mc², where R_∞ is the Rydberg constant, h is Planck's constant, c is the speed of light, α is the fine-structure constant, and m is the mass of the electron; numerically, it is approximately 27.21 electronvolts or 4.360 × 10^-18 joule. Also known as Hartree energy.

[Etymology: D. R. Hartree; UK 1897-1958] sub-atomic physics Originally applied as a qualifier to any unit in a set introduced by Hartree;
[Hartree D. R. Proc. Camb. Phil. Soc. Vol. 24, 89 (1927)] now, as Hartree energy (symbol E_h), specifically attached to that for energy. Called by Hartree the double Rydberg, it is the energy level below which certain effects cease, = 4.359 744 17(75) × 10^-18 J with relative standard uncertainty 1.7 × 10^-7.
[Mohr P. J., Taylor B. N. CODATA Recommended Values of the Fundamental Physical Constants: 2002 (to be published)]
[Mohr P. J., Taylor B. N. Rev. Mod. Phys. Vol. 72:351-495 (2000)]
[Mohr P. Phys. Today Vol. 53:7, 11-16 (2000)]
[For latest recommended values, see http://physics.nist.gov/cuu/Constants/index.html] The atomic unit of energy.