Computational atomic physics continues to play a crucial role in both increasing the understanding of fundamental physics (e.g., quantum electrodynamics and correlation) and producing atomic data for interpreting observations from large-scale research facilities ranging from fusion reactors to high-power laser systems, space-based telescopes and isotope separators. A number of different computational methods, each with their own strengths and weaknesses, is available to meet these tasks. Here, we review the relativistic multiconfiguration method as it applies to the General Relativistic Atomic Structure Package [grasp2018, C. Froese Fischer, G. Gaigalas, P. Jönsson, J. Bieroń, Comput. Phys. Commun. (2018). DOI: 10.1016/j.cpc.2018.10.032]. To illustrate the capacity of the package, examples of calculations of relevance for nuclear physics and astrophysics are presented.

Grasp is a software package in Fortran 95, adapted to run in parallel under MPI, for research in atomic physics. The basic premise is that, given a wave function, any observed atomic property can be computed. Thus, the first step is always to determine a wave function. Different properties challenge the accuracy of the wave function in different ways. This software is distributed under the MIT Licence.

We summarize here the results of a major eight-year investigation of the extraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic, main-sequence, chemically peculiar star chi Lupi (B9.5p HgMn + A2 Vm). The UV observations are composed of 345 Angstrom of the spectrum acquired with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope at an average resolution of 0.023 Angstrom. The complete set of echelle spectrograms is presented as an atlas in a companion paper. These data were supplemented by optical-wavelength spectra obtained at the Angle-Australian Telescope. Quantitatively accurate analysis and theoretical interpretation of these data required major improvements in the accuracy and completeness of available atomic data-wavelengths, transition probabilities, hyperfine structure, and isotope shifts-for the lowest ionization states of many elements. A large, international group of theoretical and experimental atomic physicists has collaborated in this investigation, and their results are summarized or referenced in this paper. In turn, the GHRS observations of chi Lupi have become a useful source of data for atomic spectroscopy, displaying many transitions that are difficult to observe in a laboratory setting. Measured abundances or upper limits are presented for 72 ions of 51 chemical elements, spanning the periodic table. We have confirmed and refined previously identified isotopic abundance anomalies in mercury and platinum and have discovered similar isotopic anomalies in thallium and, tentatively, in lead. Large discrepancies among the LTE abundances derived, using a chemically homogeneous model atmosphere, from two or three ionization states of the same element are found to be common. In some cases these are due to departures from LTE in the ionization equilibria, but the largest such discrepancies probably result from chemical stratification within the photosphere. We find qualitative trends in the abundances of the elements that clearly signify radiatively driven diffusion and gravitational settling as the primary mechanism producing abundance anomalies. However, detailed non-LTE diffusion calculations for mercury and thallium show that there is insufficient unsaturated radiative force within the chemically enriched atmosphere to sustain the observed huge overabundances of these elements in equilibrium with gravity. Either other hydrodynamic processes, such as slow mass motions or unexpectedly strong stellar winds must assist radiation pressure in supporting the enriched material, or the observed abundance patterns simply provide a snapshot in time of a nonequilibrium, time-variable phenomenon