Mixed uranyl aquo fluoro complexes [UO2(H2O)(x)F-y](2) (y = 1-4; x + y = 4, 5) have been optimized with BLYP and B3LYP density functionals in vacuo and in a polarizable continuum modeling bulk water, and have been studied at the BLYP level with Car-Parrinello molecular dynamics ( MD) simulations. Using constrained MD simulations and thermodynamic integration, the computed free binding energy between aqueous uranyl and fluoride, affording [UO2(H2O)(4)F](+), is in excellent agreement with experiment. With the same technique, five-coordinate [UO2F4(H2O)](2) is indicated to be unstable against loss of the water ligand, as the free energy for dissociation is computed to be ca. -7 kcal/mol in aqueous solution. Crown Copyright (c) 2008 Published by Elsevier B. V. All rights reserved.