The NIST Reference on Constants, Units and Uncertainty

Fundamental Physical Constants
Constants
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Energy
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Basis of conversion factors for energy equivalents

Conversion factors for energy equivalents are derived from the following relations:

    E = mc2 = hc/ = hnu = kT 

    1 eV = (e/C) J

    1 u = mu = (1/12) m (12C) = 10-3 kg mol-1/NA

    Eh = 2Rhc = 2mec2.