Gross atomic structure ($E_n\sim -{\alpha }^2\mu c^2/(2n^2)$)

Gross atomic structure denotes the leading nonrelativistic Coulomb binding levels, before fine and hyperfine splittings. For a hydrogenic two-body system, the reduced mass controls the binding energy and characteristic separation. Fine structure is smaller by roughly ${\alpha }^2$; nuclear magnetic hyperfine effects have an additional electron-to-nuclear mass suppression.