The structures, energies, and charges of uranyl cation complexes with water molecules, nitrate ion, and carbonate ions were determined using Hartree-Fock, second-order Moller-Plesset (MP2) perturbation theory, and density functional theory (DFT) ab initio quantum chemical methods. Reasonable agreement with experimentally determined structures was found. Significant polarization of the ligands as well as charge transfer to the uranyl ion was observed in the complexes; The dissociation energy curves for the complexes were also determined at the MP2 level of theory. Attempts to reproduce these curves with molecular mechanical models with fixed atomic point charges failed, showing that an appropriate force field for these systems must include polarization and charge-transfer effects.