The n·aσ charge-transfer complexes formed by ammonia and bromine monochloride and bromine dimer have been studied with various approximate pure and hybrid density functional methods and the second-order Møller–Plesset (MP2) theory. The calculations demonstrate that one hybrid method, namely the so called BH&HLYP, and the MP2 method lead to reasonably good estimates of the experimentally measured rotational constants. However, the predicted BH&HLYP intermolecular distance is found to be closer to the experimental value. Our best estimation for the intermolecular interaction energies are 7.32 and 5.48kcal/mol, respectively, for bromine monochloride and bromine dimer complexes. We have also been able to calculate the number and energies of the vibrational states supported by their corresponding intermolecular stretching modes. Sixteen such states have been found for the former and only 13 for the latter. The performance of the various approximate density functionals and the MP2 theory is compared and discussed. Finally, we comment on the analysis of the natural bond orbitals, which has been found to be very valuable to shed light on the nature of weak intermolecular interactions.