How well can density functional methods describe hydrogen bonds to π acceptors?

We employed four newly developed density functional theory (DFT) methods for the calculation of five π hydrogen bonding systems, namely, H ₂O-C ₆H ₆, NH ₃-C ₆H ₆, HCl-C ₆H ₆, H ₂O-indole, and H ₂O-methylindole. We report new coupled cluster calculations for HCl-C ₆H ₆ that support the experimental results of Gotch and Zwier. Using the best available theoretical and experimental results for all five systems, our calculations show that the recently proposed MPW1B95, MPWB1K, PW6B95, and PWB6K methods give accurate energetic and geometrical predictions for π hydrogen bonding interactions, for which B3LYP fails and PW91 is less accurate. We recommend the most recent DFT method, PWB6K, for investigating larger π hydrogen bonded systems, such as those that occur in molecular recognition, protein folding, and crystal packing.