Binding energy of an off-center shallow donor D- in a spherical quantum dot

The binding energy of a negatively charged hydrogenic impurity with on- and off-center position in a spherical Gaussian quantum dot was calculated with the configuration interaction method. Our calculations show that $E_b$ is always positive for on-center impurities with a maximum near to the radius for one-electron stability of the potential well $R_c$. For off-center positions the binding energy can assume negative values within a range of the quantum dot radius, thus indicating the instability of the system.