Abstract
Although heavy rare-earth hydrides have been widely predicted to host high-Tc superconductivity, the predicted pairing strength depends sensitively on the representation of 4f electrons within pseudopotential-based density functional perturbation theory (DFPT) calculations. We use cubic REH6 (RE = Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu) as a controlled benchmark to compare frozen-4f and explicit-4f valence descriptions across the 100–250 GPa pressure range. Frozen-4f calculations render the entire TbH6–LuH6 series qualitatively YH6-like, yielding uniformly elevated Allen–Dynes transition temperatures. By contrast, explicit-4f calculations reshape the electronic states at the Fermi level and substantially suppress the electron–phonon coupling for the open-shell members, TbH6–ErH6. Direct spin-polarized DFPT leaves the harmonically stable TbH6 and DyH6 states weakly coupled and renders HoH6 dynamically unstable, while response-derived DFT+U favors magnetic primitive-cell solutions throughout TbH6–TmH6. Beyond this subset, TmH6 is dynamically stable only at 100 GPa on the sampled pressure grid, with a low Tc, whereas YbH6 is stable only at the sampled 250 GPa point, with an intermediate transition temperature. Mode-resolved linewidths and the combined electron localization function (ELF) and density of states (DOS) descriptors reveal that hydrogen-cage connectivity alone is insufficient to sustain strong coupling. The H-derived Fermi-level weight and its coupling to H-cage phonons must also be preserved. Consistent with this mechanistic picture, LuH6, the closed-shell endpoint of the series, retains dynamical stability and strong electron–phonon coupling at 200–250 GPa because an itinerant H-1s/rare-earth-5d channel is recovered. This result shows that open-shell 4f contributions are not required for high-Tc pairing in this reference structure. Hubbard-corrected bands and Bader charge partitioning further show that the explicit 4f manifold is a correlation-sensitive localized channel that modifies rare-earth–hydrogen charge transfer. These findings establish pseudopotential valence treatment, magnetic and harmonic lattice stability, and orbital character at the Fermi level as essential reproducibility checks for theoretical superconductivity predictions in heavy rare-earth hexahydrides.