**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, which one might name **Hartree atomic units** 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:

- electron mass ;
- elementary charge ;
- reduced Planck's constant ;
- Coulomb's constant .

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.

Read more about Atomic Units: Use and Notation, Fundamental Atomic Units, Related Physical Constants, Derived Atomic Units, SI and Gaussian-CGS Variants, and Magnetism-related Units, Bohr Model in Atomic Units, Non-relativistic Quantum Mechanics in Atomic Units, Comparison With Planck Units

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