Relativistic central-field Greenʹs functions for the Ratip package Original Research Article

From perturbation theory, Greenʹs functions are known for providing a simple and convenient access to the (complete) spectrum of atoms and ions. Having these functions available, they may help carry out perturbation expansions to any order beyond the first one. For most realistic potentials, however, the Greenʹs functions need to be calculated numerically since an analytic form is known only for free electrons or for their motion in a pure Coulomb field. Therefore, in order to facilitate the use of Greenʹs functions also for atoms and ions other than the hydrogen-like ions, here we provide an extension to the Ratip program which supports the computation of relativistic (one-electron) Greenʹs functions in an—arbitrarily given—central-field potential image. Different computational modes have been implemented to define these effective potentials and to generate the radial Greenʹs functions for all bound-state energies image. In addition, care has been taken to provide a user-friendly component of the Ratip package by utilizing features of the Fortran 90/95 standard such as data structures, allocatable arrays, or a module-oriented design.