A simplified model of the hardness kernel, eta((r) over right arrow,(r) over right arrow (1)) congruent to delta((r) over right arrow - (r) over right arrow (1)), has been evaluated for the calculation of the global hardness in a series of Lewis acids and bases using the MP2 method. It is found that this simple model yields the appropriate ordering of the molecules according to their hardness values when compared to experimental available results. (c) 2005 Elsevier B.V. All rights reserved.