All-electron and effective core potential (ECP) MP2 ab initio calculations using several basis sets, from Pople's and Dunning's laboratories, were carried out for studying the series of weakly bound complexes: H3P ... HBr, H3P ... HCl, H3P ... Br-2, H3P ... BrCl, and H3N ... BrCl. Results from all-electron calculations are almost matched when Stuttgart ECPs are used to represent the halogen atoms and the 6-311G(2df,p) or aug-cc-pVDZ basis sets are employed for the rest of elements. Experimental values of inter- and intramolecular charge transfer are reasonably well predicted by population analyses and other theoretical methods based on the analysis of ab initio wavefunctions. The stronger nature of the H3N ... BrCl interaction when compared with H3P ... BrCl is rationalized in terms of the balance of physically meaningful contributions to the interaction energy. It is demonstrated that the universally accepted correlation between charge transfer and binding strength can be affected by the presence of repulsive forces arising from the Pauli exclusion principle. (C) 2003 Elsevier Science B.V. All rights reserved.