Characterizing the Potential Energy Surface of the Water Dimer with DFT: Failures of Some Popular Functionals for Hydrogen Bonding

Ten stationary points on the water dimer potential energy surface have been examined with ten density
functional methods (X3LYP, B3LYP, B971, B98, MPWLYP, PBE1PBE, PBE, MPW1K, B3P86, and
BHandHLYP). Geometry optimizations and vibrational frequency calculations were carried out with the TZ2P(f,d)+dif basis set. All ten of the density functionals correctly describe the relative energies of the ten stationary
points. However, correctly describing the curvature of the potential energy surface is far more difficult. Only
one functional (BHandHLYP) reproduces the number of imaginary frequencies from CCSD(T) calculations.
The other nine density functionals fail to correctly characterize the nature of at least one of the ten (H2O)2
stationary points studied here.