Exchange and correlation energies of ground states of atoms and molecules in strong magnetic fields

Using a Hartree-Fock mesh method and a configuration-interaction approach based on a generalized Gaussian basis set, we investigate the behavior of the exchange and correlation energies of small atoms and molecules, namely, the helium and lithium atoms as well as the hydrogen molecule, in the presence of a magnetic field covering the regime $B=0\mathrm{}–100\hspace{0.5em}\mathrm{a}.\mathrm{u}.\mathrm{}$ In general, the importance of the exchange energy to the binding properties of atoms or molecules increases strongly with increasing field strength. This is due to the spin-flip transitions, and in particular due to the contributions of the tightly bound hydrogenic states which are involved in the corresponding ground states of different symmetries. In contrast to the exchange energy the correlation energy becomes less relevant with increasing field strength. This holds for the individual configurations constituting the ground state and for the crossovers of the global ground state.