Configuration interaction calculations for two-electron atoms in a spherical box

We report calculations of the ground state (1S) correlation energy for the two electron systems He, Li+, and Be2+ when they are compressed in a spherical box with infinite potential barriers. The calculations were performed using a 41 term configuration interaction expansion generated by a 6s4p4d orbital basis. It is found that although there is an increase in the correlation energy when the sphere radius becomes smaller, this increase is extremely small with respect to the change in total energy. This fact is discussed within the context of the z-1 expansion for the energy. We also discuss the similarity between the compression process induced on an atom by increasing its nuclear charge and by the process employed in this work