Dimers and trimers of formamidine and its mono-halogenated analogues HNCHNHX, (XÂ =Â H, Cl, Br, or I): A comparative study of resonance-assisted hydrogen and halogen bonds

► NX⋯N interactions are studied in dimers and trimers of formamidine (X = H) and halogenated analogues (X = halogen). ► Trimerization strengthens the halogen bonds, but weakens the hydrogen bonds. ► NH⋯N interactions are stronger than NCl⋯N and NBr⋯N interactions. ► For dimers, NI⋯N interactions are as strong as NH⋯N interactions. ► For trimers, NI⋯N interactions are the strongest, with symmetric N⋯I⋯N interactions.