# Atomic units

**Atomic units**(au) are a convenient

system of units of measurement used in

atomic physics, particulary for describing the properties of

electrons. The atomic units have been chosen such that the fundamental electron properties are all equal to one atomic unit.

The atomic units are:

- length: the Bohr radius
- mass: the electron rest mass
- charge: the elementary charge
- energy: the Hartree energy

The use of atomic units also simplifies the

Schrödinger equation. For example the

Hamiltonian for an electron in the

hydrogen atom would be:

in SI units:

in

**atomic units**:

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