Atomic units

The atomic units, abbreviated a.u. is a set of units used in atomic calculations. In the a.u. system any four of the five quantities charge e, mass me, action ℏ, length a0, and energy Eh may be taken as base quantities, and other quantities are derived. In particular, time is a derived quantity, ℏ/Eh, with the interpretation as the period of the first Bohr orbit divided by 2&pi;. Because length is a basic unit, the speed of light is a measured quantity in a.u., c=1/&alpha; where the fine structure constant &alpha; = 1/137.035 999 11(46).

Units
Here, c = speed of light and &alpha; = fine structure constant.