Improved Thomas-Fermi interaction potential for atoms with very dissimilar nuclear charges

The interaction potential, U(R), for atoms with nuclear charges Z1 and Z2 (Z1>>Z2) is discussed. It is shown that within the linear approximation in Z2/Z1 this potential is determined by the atomic screening function, phi(R; Z), alone. On the basis of the Thomas-Fermi model allowing for quantum corrections, several universal expressions for phi(R; Z) are suggested. They permit reliable estimates of U(R) up to Rproportional2 a.u. and for Z2/Z1< or approx.1/5. (orig.)