FitAik: A package to calculate least-square fitted atomic transitions probabilities

•A new method and a new computer code called FitAik, to calculate atomic Einstein coefficients have been developed.•This code is interfaced with the well-known atomic-structure package by R. D. Cowan.•It is well suited for atoms and ions with a complex structure as lanthanides.•Detailed results are presented in the case of the Er+ ion.•The calculated Einstein coefficients and oscillator strengths are published in the new database CaDDiAcS. We present a new method implemented in our new package FitAik, to perform least-squares fitting of calculated and experimental atomic transition probabilities, by using the mono-electronic transition integrals 〈nℓ|r|n′ℓ′〉 (with r the electronic radial coordinate) as adjustable quantities. FitAik is interfaced to the Cowan suite of codes, for which it automatically writes input files and reads output files. We illustrate our procedure with the example of Er+ ion, for which the agreement between calculated and experimental Einstein coefficients is found to be very good. The source code of FitAik can be found on GitLab, and the calculated Einstein coefficients are stored in our new database CaDDiAcS. They are also used to calculate the dynamic dipole polarizability of Er+.