Scalar relativistic and spin-orbit effects in closed-shell superheavy-element monohydrides

Relativistic and electron correlation effects are investigated for the closed-shell superheavy-element monohydrides RgH, 112H⁺, 113H, 114H⁺, 117H, 118H⁺, 119H, and 120H⁺. Periodic trends are discussed by comparing the calculated properties to the ones for the lighter elements. The size of the relativistic effects varies considerably between the different molecules with the s-block elements being dominated by scalar relativistic effects and the p-block elements dominated by spin-orbit effects. In most cases, relativistic effects are more important than electron correlation effects and both are nonadditive as one expects. 120H⁺ behaves in a counterintuitive way as it shows a relativistic bond-length expansion together with a large relativistic decrease in the dissociation energy. The reason behind this anomalous behavior is due to the relativistically diminished valence-$7d$ participation in the 120-H bond.