The binding energy of an electron bound to two ionized impurity atoms is calculated as a function of the screening parameter $\delta$, which occurs in the screeened Coulomb potential. The binding energy is found to diminish with increase in $\delta$, ultimately becoming zero at $\delta \approx 1.254$. The role played by such a system in the Mott transition is discussed.

• Received 29 March 1971

DOI:https://doi.org/10.1103/PhysRevB.4.4452