H-bonded complexes of p-X-PhOH/p-X-PhO− with fluoride and hydrofluoric acid (X = OH, H, NO2) were subject of optimization (by means of B3LYP/6-311+G**) for gradually changed O···F distance from dO···F = 4.0 Å down to (i) the distance of the proton transfer from the hydroxyl group to fluoride leading to O−···HF interaction and (ii) fully optimized system (O−···HF type). In this way, we simulate gradual changes of H-bond strength estimating simultaneously the energy of interaction, Eint, energy of deformation, Edef, and the binding energy, Etot. The obtained geometrical parameters allow us to show that H-bond formation causes substantial changes in geometry, even at so distant parts of the system as the ring and bond length in para-substituents (OH and NO2). All these changes are monotonically dependent on interaction and deformation energies. Copyright © 2009 John Wiley & Sons, Ltd.