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Atomic units

Atomic units(au) are a convenient system of units of measurement used in atomic physics, particularly 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 a0
mass: the electron rest mass me
charge: the elementary charge e
energy: the Hartree energy Eh
angular momentum: the Dirac constant \hbar


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:

\hat H ={{{-\hbar^2} \over {2 m_e}}\nabla^2} + {1 \over {4 \pi \epsilon_0}}{{-e^2} \over {r}}

in atomic units:

\hat H ={{{-1} \over {2}}\nabla^2} + {{-1} \over {r}}

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