Exact Computation of the van der Waals Constant for Two Hydrogen Atoms

An exact formula is obtained for ${C}_{\mathrm{HH}}$, the van der Waals constant which appears in ${V}_{\mathrm{HH}}=\ensuremath{-}\frac{{C}_{\mathrm{HH}}}{{R}^{6}}$, the London potential for two hydrogen atoms. Numerical evaluation gives ${C}_{\mathrm{HH}}=6.4990297\ifmmode\pm\else\textpm\fi{}0.0000004$.