RELCI: A program for relativistic configuration interaction calculations

The set-up and diagonalization of (large) Hamiltonian matrices are two key elements in studying the structure and properties of many-electron atoms and ions. The efficiency in dealing with these tasks eventually determines for which atomic systems useful ab initio predictions can be made today and how accurate these predictions are. To facilitate further structure calculations, in particular for open-shell atoms and ions, here we present a new configuration interaction program in the framework of the Ratip package which help incorporate different approximations to the electron-electron interaction in the Hamiltonian matrix and, thus, into the representation of the wave functions. Our new program also supports several computational modes to allow for a flexible choice between particular time and storage requirements of the user. Care has been taken to provide a modern and user-friendly component of the Ratip package which carefully applies the concepts of Fortran 90/95.