The b pi.a sigma charge-transfer complex formed by ethyne and chlorine monofluoride has been studied with various approximate pure and hybrid density functional methods and the second-order Moller-Plesset (MP2) theory. The calculations demonstrate that one hybrid method, namely, the so-called B3LYP, leads to reasonably good estimates of the experimentally measured rotational constants. Accordingly, the predicted B3LYP intermolecular distance is found to be also close to the experimental value. This enables us to estimate reliably the intermolecular interaction energy, which is found to be 3.43 kcal/mol. 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 the weak intermolecular interaction.