The units designed to simplify the form of the fundamental equations of quantum mechanics by eliminating from them fundamental constants. The atomic unit of length is the Bohr radius, ao = h2/4π2.m.e2 = 5.29177249E-11 m (0.529177249 Å). Energy is measured in hartrees, where 1 hartree = e2/ao = 4.3597482 x 10E-18 J. Masses are specified in terms of atomic mass unit, amu = 1.6605402 x 10E-27 kg and of the electron mass unit, me = 0.910953 x 10E-30 kg. The advantage of atomic units is that if all calculations are directly expressed in such units, the results do not vary with any revision of the numerical values of the fundamental constants.