Fine-Tuning of Atomic Energies in Relativistic Multiconfiguration Calculations

Abstract

Ab initio calculations sometimes do not reproduce the experimentally observed energy separations at a high enough accuracy. Fine-tuning of diagonal elements of the Hamiltonian matrix is a process which seeks to ensure that calculated energy separations of the states that mix are in agreement with experiment. The process gives more accurate measures of the mixing than can be obtained in ab initio calculations. Fine-tuning requires the Hamiltonian matrix to be diagonally dominant, which is generally not the case for calculations based on (Formula presented.) -coupled configuration state functions. We show that this problem can be circumvented by a method that transforms the Hamiltonian in (Formula presented.) -coupling to a Hamiltonian in (Formula presented.) -coupling for which fine-tuning applies. The fine-tuned matrix is then transformed back to a Hamiltonian in (Formula presented.) -coupling. The implementation of the method into the General Relativistic Atomic Structure Package is described and test runs to validate the program operations are reported. The new method is applied to the computation of the (Formula presented.) transitions in C III and to the computation of Rydberg transitions in B I, for which the (Formula presented.) perturber enters the (Formula presented.) series. Improved convergence patterns and results are found compared with ab initio calculations.