Using CI embedding method, we have studied the adsorption of CO on MgO(100). The MgO(100) substrate is described by a Mg9O9 (3 x 3 x 2) core cluster, embedded in ionic (Mg2+/O-2) core potentials. The adsorption energy is calculated to be 0.11 eV at the CI level with a blue shift of 19 cm (1) for CO stretching on MgO(100). The dispersion accounts only 35% of the total binding energy of CO on MgO(100). The CO/MgO(100) interaction is weak and mainly of the van der Waals type with only slight chemical bonding characters. (c) 2008 Elsevier B. V. All rights reserved.