An Investigation of the Accuracy of Different DFT Functionals on the Water Exchange Reaction in Hydrated Uranyl(VI) in the Ground State and the First Excited State

We discuss the accuracy of density functional theory (DFT) in the gas phase for the water-exchange reactions in the uranyl(VI) aqua ion taking place both in the electronic ground state and in the first excited state (the luminescent 3Δg state). The geometries of the reactant and intermediates have been optimized using DFT and the B3LYP functional, with a restricted closed-shell formalism for the electronic ground state and either an unrestricted open-shell formalism or the time-dependent DFT method for the 3Δg state. The relative energies have been computed with wave-function-based methods such as Møller–Plesset second-order perturbation theory, or a minimal multireference perturbative calculation (minimal CASPT2); coupled-cluster method (CCSD(T)); DFT with B3LYP, BLYP, and BHLYP correlation and exchange functionals; and the hybrid DFT−multireference configuration interaction method. The results obtained with second-order perturbative methods are in excellent agreement with those obtained with the CCSD(T) method. However, DFT methods overestimate the energies of low coordination numbers, yielding to too high and too low reaction energies for the associative and dissociative reactions, respectively. Part of the errors appears to be associated with the amount of Hartree–Fock exchange used in the functional; for the dissociative intermediate in the ground state, the pure DFT functionals underestimate the reaction energy by 20 kJ/mol relative to wave-function-based methods, and when the amount of HF exchange is increased to 20% (B3LYP) and to 50% (BHLYP), the error is decreased to 13 and 4 kJ/mol, respectively.