The conformational properties of five gadolinium(III) complexes with polyamino carboxylate (PAC) ligands used as magnetic resonance imaging contrast agents have been investigated by ab initio and molecular mechanics (MM) methods. Ah initio calculations were performed using an effective core potential (ECP) that includes 4f electrons in the core and an optimized valence basis set for the metal. To test the reliability of ECP calculations, full geometry optimizations of Gd complexes were performed at the RHF and DFT (B-LYP) levels using the 3-21G and the 6-31G* basis sets for the ligands. Comparison with experimental data shows that ab initio calculations provide quite accurate geometries and correct conformational energies at the RHF level. Within the framework of a valence force fields, parameters for Gd-ligand interactions were determined by fitting the empirical potential to the ab initio potential energy surface (PES) of the [Gd-DOTA(H2O)](-1) complex. Sampling of the PES was performed by moving the ion into the frozen coordination cage of the ab initio optimized geometry; for each generated structure the energy and first derivatives, with respect to the Cartesian coordinates of the metal and donor atoms, were calculated at the RHF level using both 3-21G and 6-31G* basis sets for the ligand. For each considered basis set, two sets of parameters, with the electrostatic contribution turned on or off in the force fields, were determined. All the implemented sets of parameters provide reliable molecular geometries for PAC complexes, but only sets derived including the electrostatic contribution correctly reproduce the observed trend of conformational energies.