High-level G4 ab initio calculations have been carried out to investigate the binding in NH3:BeH2−nXn (X = F, Cl, Br; n ⩽ 2) complexes. The results obtained show that these dimers are stabilized by quite strong beryllium bonds, which are accompanied by a dramatic distortion of the BeH2−nXn Lewis acids, which depart noticeably from linearity whereas the BeH and BeX bonds become significantly longer. This deformation plays a crucial role as far as the relative stability trends are concerned since it amounts to about a 30% of the total interaction energy. More importantly, this deformation significantly enhances the electron acceptor capacity of the Lewis acid, which follows completely different trends from those expected for the undistorted systems. Consistently, a frontier orbital theory is able to explain the trends observed in the interaction energies, provided that it is carried out on the distorted systems rather than on the systems in their equilibrium conformation.