Table 18.

Comparison of B3LYP and PM6 hydrogen bond energies (kcal mol⁻¹)

Hydrogen-bonded system	Ref	PM6	PM5	PM3	AM1
Ammonia - ammonia	−2.94	−2.34	−0.77	−0.67	−1.41
Water - methanol	−4.90	−5.12	−2.59	−0.20	−4.52
Water - acetone	−5.51	−5.25	−2.43	−2.22	−4.09
Water, dimer, linear (O–H–O = 180°)	−5.00	−3.69	−1.57	−3.49	−3.16
Water, dimer	−5.00	−4.88	−2.43	−1.95	−5.01
Benzene dimer, T-shaped	−2.34	−0.83	−0.22	−0.56	−0.07
Water - acetate anion	−19.22	−18.72	−12.28	−15.77	−15.91
Water - formaldehyde	−5.17	−4.22	−2.17	−2.73	−3.40
Water - ammonia	−6.36	−4.32	−2.75	−1.53	−2.90
Water - formamide	−8.88	−7.60	−4.14	−4.33	−7.54
Formic acid, dimer	−13.90	−10.03	−4.75	−8.65	−6.44
Water - methylammonium cation	−18.76	−14.90	−8.94	−10.48	−14.36
Formamide - formamide	−13.55	−10.83	−4.46	−6.08	−8.14
Acetic acid, dimer	−14.89	−10.33	−4.50	−8.70	−6.44