To improve the description of Pu(III)/Pu(IV) hydration and redox thermodynamics, for plutonium, the PBE0-D4 functional is used with the MBW60-SEG valence electron basis set and pseudopotentials, while the hydrogen and oxygen atoms in water are treated with the def-TZVP basis set. Both an 8-water first-shell model and a 30-water cluster containing eight first-shell and twenty-two second-shell waters were examined. Gas-phase and CPCM(water) calculations were performed separately; the 8-water gas structures were used as the reference geometries, whereas the 30-water CPCM energies were used for the double-shell thermodynamic comparison. The revised 8-water gas-phase benchmark gives mean Pu–O distances of 2.51 and 2.39 Å for Pu(III) and Pu(IV), respectively, only 0.02 Å from the selected EXAFS references of 2.53 and 2.41 Å. In the 30-water solution-phase model, the corresponding mean distances are 2.47 and 2.34 Å. The calculated hydration Gibbs free energies for trivalent plutonium, tetravalent plutonium, and their clusters with 30 water molecules are −3241.68 and −6202.49 kJ·mol⁻¹. The Pu(IV/III) reduction potential is 1.14 V, only 0.09 V (8.57%) above the experimental value of 1.05 V and 0.34 V below the reported double-shell COSMO/B3LYP value of 1.48 V. Explicit second-shell hydration lowers the PBE0 potential from 2.73 V for 8 H₂O to 1.14 V for 30 H₂O.

Plutonium; Density functional theory; Hydration structure; Hydration Gibbs free energy; Redox thermodynamics; CPCM