Relativistic coupled-cluster-theory analysis of unusually large correlation effects in the determination of gj factors in Ca+

We investigate roles of electron correlation effects in the determination of the gj factors of the 4s2S1/2, 4p2P1/2, 4p2P3/2, 3d2D3/2, and 3d2D5/2 states, representing different parities and angular momenta, of the Ca+ ion. Correlation contributions are highlighted with respect to the mean-field values evaluated using the Dirac-Hartree-Fock method, relativistic second-order many-body theory, and relativistic coupled-cluster (RCC) theory with the single- and double-excitation approximation considering only the linear terms and also accounting for all the nonlinear terms. This shows that it is difficult to achieve results below 10-5 precision employing an approximate perturbative approach. We also find that contributions through the nonlinear terms and higher-level excitations such as triple excitations, estimated perturbatively in the RCC method, are found to be crucial to attain precise values of the gj factors in the considered states of the Ca+ ion.