Atomic fundamental physical constant used as atomic unit of energy: Eh = ℏ2/me.a20 = 4.359 7482(26) E-18 J, where ℏ is the Planck constant divided by 2π, me the electron rest mass and a0 the Bohr radius.
Source:
CODATA Bull. 1986, 63, 1 (https://doi.org/10.6028/jres.092.010)