Density functional theory calculations on the benzene-ammonia complex have been performed. The benzene-ammonia complex is weakly hydrogen bonded, and crucial for obtaining good agreement with experimental results is the choice of the exchange functional used, as in the case of van der Waals interactions. It is found that the behavior of the exchange functional in the region where the density is small and the density gradient large plays an important role in describing the weak hydrogen-bonding interaction. Calculations show that generalized gradient approximation functionals, which behave properly in this region, have achieved an accuracy similar to the second-order Moller-Plesset (MP2) method for the benzene-ammonia complex.