Adsorption of CO on Pt(111) has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full-geometry optimization, changing calculation parameters (core treatment, functional, number of slab layer, k-points, and size of unit cell). There is a puzzling problem for the CO/Pt(111) system that DFT tends to predict the wrong adsorption site, the fcc-hollow site, whereas experimentally the atop site is preferred. The present work clearly shows that all electron scalar relativistic (AER) calculations are essential to obtain the correct site-preference, atop followed by bridge and then hollow (fcc and hcp). The AER calculations give a deeper Fermi level for a Pt surface with all the functionals, which provides the effect for decreasing the interaction of the LUMO of CO with the metal substrate. The adsorption structures agree well with those determined by LEED. (C) 2003 Elsevier B.V. All rights reserved.