The Laplacian of electron density is employed in the description of halogen bonds between some halogen containing molecules, A-X, and ammonia. We show that a halogen bond is an interaction between a region of charge depletion (a hole) on the halogen atom and a region of charge concentration (a lump) on another molecule. The existence of the hole on the Valence Shell Charge Concentration (VSCC) of halogen atom is a necessary condition in the formation of halogen bonds. In addition, for some chlorine containing molecules, it is shown that the position and properties of the hole determines the geometry and strength of the halogen bond. (C) 2010 Elsevier B.V. All rights reserved.