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 Moller-Plesset perturbation theory provide a reasonable description, and explicit inclusion of multi-configurational effects involving Pu 5f-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. (C) 2012 Elsevier B.V. All rights reserved.