Electronic structures of hydrate complex models of Th4+ and Ac3+ ions (Th(H2O)n(4)(+) and Ac(H-2)(n)(3+) (n = 1,2,4,6)) were investigated by all-electron four-component relativistic calculations based on the Dirac-Hartree-Fock (DHF) method. Picture of the coordinate bond in the complexes was observed, and the difference between Th4+ and Ac3+ cases was shown. Relativistic effective-core potential (RECP) calculations were also performed on the higher hydrate models to assess the bulk solvation effect by using the conductive polarizable continuum model (CPCM) reaction-field scheme. (C) 2003 Elsevier Science B.V. All rights reserved.