Periodic ab initio B3-LYP calculations on the MgO(0 0 1)/CO system underestimate the CO binding energy value with respect to experiment. The flaw is in the B3-LYP functional, unable to compute dispersive interactions. Here we show how to evaluate this contribution by adopting a two-layer ONIOM scheme in which the B3-LYP crystal energy is improved by the MP2 energy computed on the Mg9O9/CO cluster. The final complete basis set extrapolated (MP2/infinity:B3-LYP/VTZ) CO/Mg(0 0 1) binding energy of 13 kJ/mol is in good agreement with experiment, with about 7 kJ/mol deriving from dispersive interactions. (C) 2002 Elsevier Science B.V. All rights reserved.