Long distance interactions of ammonia, hydrogen fluoride and water molecules adsorbed on a nickel (111) surface have been investigated using density functional theory (DFT) and a classical electrostatic model. The DFT approach uses periodic boundary conditions, the generalized gradient approximation and plane wave basis functions. In the classical approach, the molecules are treated as point dipoles and the metal surfaces are modelled using the image-charge method. The classical and DFT interaction energies agree well, and the image-charge method can thus be used as a simple description of interactions between molecules and metal surfaces. (C) 2009 Elsevier B. V. All rights reserved.