Ab initio structure and energetics of Pu(OH)4 and Pu(OH)4(H2O)n clusters: Comparison between density functional and multi-reference theories

The electronic and geometric structure of the Pu(OH)4 molecule and Pu(OH)4(H2O)n ( n = 1 - 19 ) clusters are compared using a range of single- and multi-reference theories. We find that single-reference methods such as unrestricted, second-order Møller–Plesset perturbation theory provide a reasonable description, and explicit inclusion of multi-configurational effects involving Pu 5 f -electrons is not essential. However, density functional theory (DFT) with standard approximations for exchange–correlation performs poorly for Pu(OH)4 and Pu(OH)4(H2O)n clusters. We propose the use of DFT + U as a simple improvement over standard DFT, and determine an ab initio parameterization of DFT + U suitable for atomistic simulations of Pu(OH)4 in aqueous environments.