Structures of uranyl hydroxide monomeric UO2(OH)(2)(0) and dimeric (UO2)(2)(OH)(2)(2+) were studied by hybrid density functional theoretical calculations. It was found out that both the monomeric ortho-UO2(OH)(2)(0) and dimeric (UO2)(2)(OH)(2)(2+) complexes have bent O=U=O angles (similar to 170 degrees). This is attributed to significant pi donation from the equatorial OH- ligands to the uranyl moiety. It was also found that the ortho- and meta-UO2(OH)(2)(H2O)(3)(0) complexes lie very close in energy, and that both isomers may exist in normal aqueous media. @ 2001 Elsevier Science BN. All rights reserved.