Assessment of Coupled Cluster Theory and more Approximate Methods for Hydrogen Bonded Systems

To assess the accuracy of post-Hartree–Fock methods like CCSD­(T), MP3, MP2.5, MP2, SCS-MP2, SOS-MP2, and DFT-SAPT, we evaluated several effects going beyond valence-correlated CCSD­(T). For 16 small hydrogen bonded systems, CCSD­(T) achieves an RMS error of 0.17 kJ/mol in the dissociation energy compared to our best estimate, which is a composite method akin to W4 theory. The error of CCSD­(T) is thus much lower than for atomization energies. MP2 is surprisingly accurate for these systems with an RMS error of 1.3 kJ/mol. MP2.5 yields a clear improvement over MP2 (RMS of 0.5 kJ/mol) but still has an error about 3 times as large as CCSD­(T) for the absolute RMS and almost 10 times as large for the relative RMS error. Neither SCS-MP2, SOS-MP2, nor DFT-SAPT yield lower errors than MP2. With a ΔCCSD­(T) correction to MP2, the basis set limit is readily achieved when employing diffuse functionswithout these, the convergence is rather slow.