Why mercury is a superconductor

Despite being the oldest known superconductor, solid mercury is mysteriously absent from all current computational databases of superconductors. In this Research Letter, we present a critical study of its superconducting properties based on state-of-the-art superconducting density functional theory. Our calculations reveal numerous anomalies in electronic and lattice properties, which can mostly be handled, with due care, by modern ab initio techniques. In particular, we highlight an anomalous role of spin-orbit coupling in the dynamical stability and of semicore d levels in the effective Coulomb interaction and, ultimately, the critical temperature.