Julian Schwinger and the Semiclassical Atom

Abstract: In the early 1980s, Schwinger made seminal contributions to the semiclassical theory of atoms. There had, of course, been earlier attempts at improving upon the Thomas--Fermi model of the 1920s. Yet, a consistent derivation of the leading and next-to-leading corrections to the formula for the total binding energy of neutral atoms, $$-\frac{E}{e^2/a_0} = 0.768745Z^{7/3} - \frac{1}{2}Z^2+0.269900Z^{5/3} + \cdots\,,$$ had not been accomplished before Schwinger got interested in the matter; here, $Z$ is the atomic number and $e^2/a_0$ is the Rydberg unit of energy. The corresponding improvements upon the Thomas--Fermi density were next on his agenda with, perhaps, less satisfactory results. Schwinger's work not only triggered extensive investigations by mathematicians, who eventually convinced themselves that Schwinger got it right, but also laid the ground, in passing, for later refinements --- some of them very recent.