When one of the H atoms of SH2 is replaced by a halogen X, the S engages in a strong S center dot center dot center dot N interaction with the N of NH3, wherein X lies directly opposite the N. The binding energy varies from 8 kcal/mol for X = F down to 5 kcal/mol for Br. This FS center dot center dot center dot N geometry represents the global minimum on the HSF/NH3 potential energy surface, more stable than minima containing either a SH center dot center dot center dot N or NH center dot center dot center dot F H-bond. It is equally stable with a SH center dot center dot center dot N structure for X = Cl, and slightly less stable than SH center dot center dot center dot N for X = Br. In all cases, the S center dot center dot center dot N minimum is more stable than geometries containing a halogen bond. (C) 2011 Elsevier B.V. All rights reserved.