Density functional theory is used to explore the trimetallic nanoparticle catalyst Ru5PtSn. Results are presented for the catalyst precursor PtRu5(CO)(15)(mu-SnPh2)(mu(6)-C), as well as for Ru5PtCSn and Ru5PtSn in the gas phase and for Ru5PtCSn supported on the (00 1) face of alpha-christobalite. Removal of the ligands introduces marked structural relaxations in the metal core. Whereas, the ligand-stabilized system has a large HOMO-LUMO gap, such a separation is not present for Ru5PtCSn and Ru5PtSn. Ru5PtCSn is predicted to be anchored to the alpha-christobalite surface via Ru-O bonds. (c) 2007 Elsevier B.V. All rights reserved.