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Atomic units (au or a.u.) form a system of natural units which is especially convenient for atomic physics calculations. There are two different kinds of atomic units, Hartree atomic units^{[1]} and Rydberg atomic units, which differ in the choice of the unit of mass and charge. This article deals with Hartree atomic units. In atomic units, the numerical values of the following four fundamental physical constants are all unity by definition:
Atomic units are often abbreviated "a.u." or "au", not to be confused with the same abbreviation used also for astronomical units, arbitrary units, and absorbance units in different contexts.
Atomic units, like SI units, have a unit of mass, a unit of length, and so on. However, the use and notation is somewhat different from SI.
Suppose a particle with a mass of m has 3.4 times the mass of electron. The value of m can be written in three ways:
These four fundamental constants form the basis of the atomic units (see above). Therefore, their numerical values in the atomic units are unity by definition.
Dimension  Name  Symbol/Definition  Value in SI units^{[5]} 

mass  electron rest mass  9.10938291(40)×10^{−31} kg  
charge  elementary charge  1.602176565(35)×10^{−19} C  
action  reduced Planck's constant  1.054571726(47)×10^{−34} J·s  
electric constant^{1}  Coulomb force constant  8.9875517873681×10^{9} kg·m^{3}·s^{2}·C^{2} 
Dimensionless physical constants retain their values in any system of units. Of particular importance is the finestructure constant . This immediately gives the value of the speed of light, expressed in atomic units.
Name  Symbol/Definition  Value in atomic units 

speed of light  
classical electron radius  
proton mass 
Below are given a few derived units. Some of them have proper names and symbols assigned, as indicated in the table. k_{B} is Boltzmann constant.
Dimension  Name  Symbol  Expression  Value in SI units  Value in more common units 

length  bohr  5.2917721092(17)×10^{−11} m^{[5]}  0.052917721092(17) nm=0.52917721092(17) Å  
energy  hartree  4.35974417(75)×10^{−18} J  27.211 eV=627.509 kcal·mol^{−1}  
time  2.418884326505(16)×10^{−17} s  
velocity  2.1876912633(73)×10^{6} m·s^{−1}  
force  8.2387225(14)×10^{−8} N  82.387 nN=51.421 eV·Å^{−1}  
temperature  3.1577464(55)×10^{5} K  
pressure  2.9421912(19)×10^{13} Pa  
electric field  5.14220652(11)×10^{11} V·m^{−1}  5.14220652(11) GV·cm^{−1}=51.4220652(11) V·Å^{−1}  
electric dipole moment  8.47835326(19)×10^{−30} C·m  2.541746 D 
There are two common variants of atomic units, one where they are used in conjunction with SI units for electromagnetism, and one where they are used with GaussianCGS units.^{[6]} Although the units written above are the same either way (including the unit for electric field), the units related to magnetism are not. In the SI system, the atomic unit for magnetic field is
and in the Gaussiancgs unit system, the atomic unit for magnetic field is
(These differ by a factor of α.)
Other magnetismrelated quantities are also different in the two systems. An important example is the Bohr magneton: In SIbased atomic units,^{[7]}
and in Gaussianbased atomic units,^{[8]}
Atomic units are chosen to reflect the properties of electrons in atoms. This is particularly clear from the classical Bohr model of the hydrogen atom in its ground state. The ground state electron orbiting the hydrogen nucleus has (in the classical Bohr model):
The Schrödinger equation for an electron in SI units is
The same equation in au is
For the special case of the electron around a hydrogen atom, the Hamiltonian in SI units is:
while atomic units transform the preceding equation into
Both Planck units and au are derived from certain fundamental properties of the physical world, and are free of anthropocentric considerations. It should be kept in mind that au were designed for atomicscale calculations in the presentday universe, while Planck units are more suitable for quantum gravity and earlyuniverse cosmology. Both au and Planck units normalize the reduced Planck constant. Beyond this, Planck units normalize to 1 the two fundamental constants of general relativity and cosmology: the gravitational constant G and the speed of light in a vacuum, c. Atomic units, by contrast, normalize to 1 the mass and charge of the electron, and, as a result, the speed of light in atomic units is a large value, . The orbital velocity of an electron around a small atom is of the order of 1 in atomic units, so the discrepancy between the velocity units in the two systems reflects the fact that electrons orbit small atoms much slower than the speed of light (around 2 orders of magnitude slower).
There are much larger discrepancies in some other units. For example, the unit of mass in atomic units is the mass of an electron, while the unit of mass in Planck units is the Planck mass, a mass so large that if a single particle had that much mass it might collapse into a black hole. Indeed, the Planck unit of mass is 22 orders of magnitude larger than the au unit of mass. Similarly, there are many orders of magnitude separating the Planck units of energy and length from the corresponding atomic units.

