Compression effects in helium-like atoms (Z=1,…,5)(Z=1,…,5) constrained by hard spherical walls

Ground and lowest triplet S state energies and other properties are obtained for confined helium-like atoms {Z=1,…,5}{Z=1,…,5} spherically enclosed by impenetrable boxes of varying size. Wave functions are variationally optimized within generalized Hylleraas bases fulfilling appropriate boundary conditions. For all systems, enhanced confinement leads to increased total energies and singlet–triplet energy splittings.