In this work, clusters consisting of two and three formamide or thioformamide molecules were subjected to ab initio and density functional theory calculations using the aug-cc-pvdz/cc-pvdz basis set. Formamide and thioformamide dimers were both found to exhibit five different minima on their potential surfaces involving hydrogen bonds of the N-H.Xdouble bondC or C-H.Xdouble bondC (Xdouble bondO, S) type. The most stable structure in both cases is a cyclic configuration of C-2h symmetry involving two identical N-H.Xdouble bondC bonds. The interaction energy for such a structure is -60 and -48 kJ/mol for formamide and thioformamide, respectively. Based on the calculations, each N-H.Xdouble bondC bond contributes -30 kJ/mol to it in formamide and -24 kJ/mol in thioformamide. On the other hand, each N-H.Xdouble bondC bond contributes -9.7 kJ/mol in formamide and -11.7 kJ/mol in thioformamide. The interaction causes appreciable distortion in the molecules, particularly in the N-H groups involved in a hydrogen bond, which are lengthened by up to 0.019 and 0.013 Angstrom in formamide and thioformamide, respectively. The trimer structures identified on the potential surfaces of formamide and thioformamide are cyclic configurations capable of establishing 3 or 4 hydrogen bonds. While formamide tends to adopt planar configurations (the most stable of which possesses an interaction energy of -105 kJ/mol), thioformamide forms preferentially nonplanar structures (the most stable being a nonplanar cyclic configuration with an interaction energy of -88 kJ/mol). The contribution of nonadditive pairwise terms is not particularly significant in either compound, which suggests the absence of substantial cooperative phenomena in the trimers. However, this contribution is crucial with a view to determining the stability sequence for the trimers, where the most stable structures result from the contribution of nonadditive pairwise terms (up to 15% of the overall interaction energy for the most stable thioformamide trimer). The interaction shifts the frequencies of modes closely involved in it. Thus, the N-H symmetric stretching frequency is redshifted by more than 300 cm(-1) and the NH2 wagging frequency is blueshifted to a similar extent. As a rule, frequency shifts are less marked in the thioformamide clusters; both substances, however, exhibit identical trends.