Molecular modeling of the large tributyl phosphate complex of europium provides a test of the sensitivity of force field calculations to molecules within the interlayer of a trioctahedral smectite clay. A nonbonded version of the consistent-valence force field was utilized. Parametrization is based on the structures of simple clays and oxides. The simulations were carried out with complete translational freedom for all atoms. Energy minimization of the hectorite supercell containing, within the interlayer, the Eu.3TBP complex as the trinitrate, eight sodium cations, and 48 waters of hydration resulted in a d spacing of 18.66 Angstrom. Molecular dynamics simulations at 298 K resulted in a d spacing of 17.40 +/- 0.15 Angstrom. The experimental d spacing for hectorite absorbed with the TBP solvate of EU(NO3)(3) is 17.18 +/- 0.04 Angstrom.