The ground state energy of a helium atom under strong confinement

We analyze the helium atom ground state under strong confinement where the atom is embedded in a spherical box with impenetrable walls. In previous studies, variational calculations of the ground state energy for strongly confined helium have proven to be a difficult numerical task, especially when large basis set expansions were considered. In the present work we have solved the time-independent Schrödinger equation using first-order Rayleigh–Schrödinger perturbation theory and then extended the calculations using fifth-order variational perturbation theory. Our results show that these approaches provide good alternative approaches for calculation of the ground state energy for a strongly confined helium atom.