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