Which functional form is appropriate for hydrogen bond of amides?

Four types of the functional form, which are the Morse, Lennard-Jones 10-12, 6-12, and 6-9 types of the potential function used frequently to describe hydrogen bonds, are tested against ab initio MP2/6-31G** calculations for their ability to describe dimerization energies of N-methylacetamide (NMA), formamide, and acetamide, varying the distance between amide hydrogen and carbonyl oxygen. All parameters for four types of the potential fitted to the scaled dimerization energies of NMA around the minimum by including counterpoise BSSE corrections at the MP2/6-31G** level reproduce satisfactorily the corrected dimerization energies of formamide and acetamide around the minima. The Morse function results in the best fit to the scaled MP2/6-31G**: energies of three amides both at short and long distances, whereas other functions are satisfactory only at long distances.