Atomic fundamental physical constant used as atomic unit of length: $a_{0}=\frac{4\ \pi \ \varepsilon_{0}\ \mathrm{\hslash }^{2}}{m_{\text{e}}\ e^{2}}=5.291 \ 772\ 49\ (24)\times 10^{-11}\ \text{m}$ where e is the elementary charge, ℏ is the Planck constant divided by 2π and m e is the electron rest mass.