The beryllium atom and beryllium positive ion in strong magnetic fields

The ground and a few excited states of the beryllium atom in external unifo rm magnetic fields are calculated by means of our 2D mesh Hartree-Fock meth od for field strengths ranging from zero up to 2.35 x 10(9) T. With changin g field strength the ground state of the Be atom undergoes three transition s 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), S-z = 0 configurati on. With increasing field strength the ground state evolves into the two S- z = -1 configurations 1s(2)2s2p(-1) and 1s(2)2p(-1)3d(-2), followed by the fully spin polarised S-z = -2 configuration 1s2p(-1)3d(-2)4f(-3). The latte r configuration forms the ground state of the beryllium atom in the high fi eld regime gamma > 4.567. The analogous calculations for the Be+ ion provid e the sequence of the three following ground state configurations: 1s(2)2s and 1s(2)2p(-1) (S-z = -1/2) and 1s2p(-1)3d(-2) (S-z = -3/2).