https://doi.org/10.1351/goldbook.B00693
Atomic fundamental physical constant used as @A00504@ 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 @E02032@, \(\mathrm{\hslash}\) is the @P04685@ divided by \(2\ \pi \) and \(m_{\text{e}}\) is the @E02008@.