  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.
