The effect of changing the L C=O -. * H on the energy of the linear hydrogen bond has been studied by ab initio calculations for one formamide-methane complex and two formamide-water complexes, which differ in the position of the second hydrogen (H2,) of the water molecule (i.e. the one not involved in the hydrogen bond). When the hydrogen bond is cis to the C-N bond (L C-O *. . H = 120"), this hydrogen is, respectively, trans (I) and cis (II) to the C-O bond. It is found that the formamide-methane complex has two minima with hydrogen bond energies of N -2.6 kcal mol-', at values of about 120" and 240" for the L c-0 * . . H. The barrier height at 180" is N 0.9 kcal mol-'. A different situation is found for the formamide-water complexes, where the hydrogen bond energy is largely dependent on the position of H2,. The formamide-water (I) complex has one minimum of -10.74 kcal mol-' for the hydrogen bond at a value of -110"for LC=O... H. The energy rises sharply with changes in the angle, and is -7.54 kcal mol-r at 180". The other complex (II) has one minimum of -9.14 kcal mol-' at 240" and a hydrogen bond energy of -7.12 kcal mol-' at 180". It is also found that the hydrogen bond length increases in a regular manner when L C=O . . . H changes from 120" to 240".