GRASPC – GRASP package adapted for the generation of continuum orbitals wave functions

The GRASP package (https://github.com/compas/grasp) is a widely used tool for performing fully relativistic bound electron structure calculations of atoms. Its latest official release is GRASP2018, but it has been continuously developed since then. The presented code, GRASPC, is the adaptation of that package allowing for calculations of the continuum orbital of electrons elastically scattered from atoms and ions. The calculated continuum orbital can be normalized using the per-energy normalization procedure. Then, the phase shifts are calculated by comparing the computed wave function with the free electron wave function in the asymptotic region. Scattering lengths are estimated not only for widely used very low energy scattering but also using an unusual approach with a “zero energy” wave function. The main idea behind GRASPC is to use as many computational apparatus as they are implemented in GRASP (e.g., building the atomic and configuration state functions, calculating the potentials, angular coefficients and integrals, constructing the Dirac-Coulomb Hamiltonian, performing self-consistent calculations) by adapting them to calculate the wave function of the scattered electron. This adaptation is entirely transparent for usual calculations in GRASP (bound states and their properties). The default flow changes only when calculations involving continuum orbital are requested and different outputs are produced. This approach, combined with the retention of the typical interactive user interface, allows GRASP users to adapt rapidly to the new type of calculation.