##### Wikipedia

**Hartree**

The **hartree** (symbol: ** Eh** or

**Ha**), also known as the

**Hartree energy**, is the atomic unit of energy, named after the British physicist Douglas Hartree. It is defined as 2

*R*

*hc*, where

*R*is the Rydberg constant,

*h*is the Planck constant and

*c*is the speed of light. The 2010 CODATA recommended value is

*E*= = . The 2006 CODATA recommended value was

*E*= = .

The hartree energy is approximately the electric potential energy of the 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) are much more widely used.