Approximate Expression for the Binding Energy of Isotropic Biexcitons

A formula which gives the binding energy of the biexciton as a function of the electron-to-hole mass ratio $\sigma$ is obtained for the case of isotropic effective masses of the carriers. The expression which contains powers of ${[\frac{\pi }{4}-arctan{\left(\sigma \right)}^{\frac{1}{2}}]}^2$ satisfies the requirements imposed by the Schrödinger equation and by the known results of variational calculations for the hydrogen and positronium molecules.