General Setup: Unless noted otherwise, computational results reported herein were obtained with the ORCA program version 5.0. Prior theoretical studies of the magnetic susceptibility of PuO 2 (see References 5 and 6 and citations therein) have shown that strong correlations of the unpaired electrons between different metal centers are not required to explain the system's electronic structure. Therefore, calculations can meaningfully be performed with the an embedded cluster model equivalent to the one used previously for the PuO 2 susceptibility study of Reference 6. Similar models have also been used successfully for MC-WFT calculations of solid CeO 2 XANES spectra or the electronic structure of solid UO 2 . The embedded cluster model for the present work utilized the 'capped' effective core potential (ECP) approach of ORCA. The model consists of three regions, namely, a PuO 8 -central moiety treated quantummechanically ('quantum cluster', QC), a surrounding boundary region (BR) of 18 Pu 4+ and 56 O 2-ions with ECP cores, and a spherical shell of radius 30 A generating a point charge (PC) field from 8624 point charges with formal +4 and -2 charges for Pu and O 2-ions, respectively. For the BR, Stuttgart-Dresden small-core ECPs were assigned. The embedding charges in the PC region were exponentially dampened at the periphery of the sphere to maintain charge neutrality of the model system, as described in Reference 6. Ion and point charge positions coincide with the experimental crystal structure. Additional calculations were carried out for water-solvated ions Pu 3+ and Pu 4+ [Pu 3+ (aq) and Pu 4+ (aq)]. These calculations used aqua complexes of the ions coordinated with nine explicit water molecules in a slightly distorted tricapped trigonal pyramid with equal Pu-O distances of 2.56 A for Pu 3+ (aq), and with three Pu-O at 2.54 A and six Pu-O at 2.44 A for Pu 4+ (aq). Additional bulk solvation effects for an aqueous environment were treated via the Conductor-like Polarizable Continuum Model (CPCM) solvation model. Similar calculations were also performed for Pu 4+ (aq) coordinated with eight explicit water molecules in a cubic arrangement at 2.34 A . The structures of the Pu ion-water clusters were taken from Reference 18 without further optimization.