Alternatively, Rayleigh-Schrödinger perturbation theory or variational approaches can be used by adding a cutoff term to the basis functions, which forces the wave function to vanish at r C , thus allowing the treatment of many-electron systems. To study diatomic molecules, the box shape is usually changed to that of a spheroid (ellipsoid), as introduced by Cottrell. The model of a hard sphere confining a quantum system does not allow for any other interaction with the surrounding than Pauli repulsion and thus can only be seen as a crude approximation of reality. As Connerade pointed out, the infinite potential wall makes the system unobservable and thus violates a basic principle of quantum mechanics. Consequently, it is not surprising that the hard-sphere confinement method cannot accurately reproduce experimental results, as for example the vibrational frequency shifts of the hydrogen molecule. As a remedy, Ley-Koo et al. proposed the use of a finite constant potential U 0 to study the hydrogen atom, such that