Hartree-Fock-Roothaan calculations for many-electron atoms and ions in neutron-star magnetic fields

The quantitative analysis of the electromagnetic spectra of isolated neutron stars by means of model atmosphere calculations requires extensive data sets of atomic energy values and transition probabilities in intense magnetic fields. We present a method for the fast computation of wave functions, energies, and oscillator strengths of medium-Z atoms and ions at neutron star magnetic field strengths B≳10⁷ T which strikes a balance between numerical accuracy and computing times. We use a Hartree-Fock ansatz in which each single-electron orbital is expanded in terms of Landau states with one longitudinal expansion function, and each Landau level contributes with a different weight to the orbital. Both the longitudinal expansion functions and the Landau weights are determined in a doubly self-consistent way. Hartree-Fock equations are solved by decomposing the z axis in finite elements and expanding the longitudinal wave functions in terms of sixth-order B-splines. The contributions of the eight lowest Landau levels are taken into account. The procedure can be efficiently parallelized. Results are presented for the ground states and different excited states of atoms and ions for nuclear charges Z=2,…,26 and N=2,…,26 electrons, and for oscillator strengths. Wherever possible, a comparison with the results of previous calculations is made.