# Hartree atomic units

• the hartree atomic units are a system of natural units of measurement which is especially convenient for atomic physics calculations. they are named after the physicist douglas hartree.[1] in this system the numerical values of the following four fundamental physical constants are all unity by definition:

• electron mass , also known as the atomic unit of mass[2][3]
• elementary charge , also known as the atomic unit of charge[4]
• reduced planck constant , also known as the atomic unit of action[5]
• vacuum permittivity , also known as the atomic unit of permittivity[6]
• boltzmann constant , also known as the atomic unit of heat capacity

in hartree atomic units, the speed of light is approximately 137.036 atomic units of speed. 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 other contexts.

• use and notation
• defining constants
• units
• physical constants
• 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