Infinite-basis calculations of binding energies for the hydrogen bonded and stacked tetramers of formic acid and formamide and their use for validation of hybrid DFT and ab initio methods.

Benchmark stabilization energies for planar H-bonded and stacked structures of formic acid tetramers and formamide tetramers were determined as the sum of the infinite basis set limit of MP2 energies and a CCSD(T) correction term evaluated with the 6-31G*(0.25) basis set. The infinite basis (IB) set limit of MP2 energies was determined by two-point extrapolation using the aug-cc-pVXZ basis sets for X = D and T and separate extrapolation of the Hartree-Fock and correlation energies with new IB parameters for augmented basis sets determined here. Final stabilization energies (kcal/mol) for the tetramer studied are in the range of 4.6 to approximately 6.7 kcal/mol and they were used as reference data to test 14 density functionals. Among the tested DFT methods, PWB6K gives the best performance with an average error equal to only 30% of the average binding energy. In contrast, the popular B3LYP functional has an average error of 85%. We recommend the PWB6K method for exploring the potential energy surfaces of organic complexes and clusters and supramolecular assemblies.