Calculations of the atomic structure for the low-lying states of actinium

We employ a technique that combines the configuration interaction method with the singles-doubles coupled-cluster method to perform calculation of the energy levels, transition amplitudes, lifetimes, $g$ factors, and magnetic dipole and electric quadrupole hyperfine structure constants for many low-lying states of neutral actinium. We find very good agreement with existing experimental energy levels and make accurate predictions for missing levels. It has been noted that some of the levels were previously misidentified; our analysis supports this claim. If spectroscopy is performed with ${}^{225}\mathrm{Ac}$, our calculations will lead to values for nuclear structure constants. The accuracy of this can be constrained by comparing with ${}^{227}\mathrm{Ac}$.