The beryllium atom and beryllium positive ion in strong magnetic fields

The ground and a few excited states of the beryllium atom in external uniform magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for field strengths ranging from zero up to $2.35\times 10^9$ T. With changing field strength the ground state of the Be atom undergoes three transitions involving four different electronic configurations which belong to three groups with different spin projections $S_z=0,-1,-2$. For weak fields the ground state configuration arises from the $1s^2 2s^2 $, Sz=0 configuration. With increasing field strength the ground state evolves into the two Sz=-1 configurations $1s^22s 2p_{-1}$ and $1s^2 2p_{-1}3d_{-2}$, followed by the fully spin polarised Sz=-2 configuration $1s2p_{-1}3d_{-2}4f_{-3}$. The latter configuration forms the ground state of the beryllium atom in the high field regime $\gamma> 4.567$. The analogous calculations for the ${\rm Be^+}$ ion provide the sequence of the three following ground state configurations: $1s^22s$ and $1s^22p_{-1}$ ($S_z=-1/2$) and $1s2p_{-1}3d_{-2}$ ($S_z=-3/2$)