A comparison of ab initio quantum-mechanical and experimental D-0 binding energies of eleven H-bonded and eleven dispersion-bound complexes
Abstract
Dissociation energies (D-0) of 11 H-bonded and 11 dispersion-bound complexes were calculated as the sum of interaction energies and the change of zero-point vibrational energies (Delta ZPVE). The structures of H-bonded complexes were optimized at the RI-MP2/cc-pVTZ level, at which deformation and harmonic Delta ZPVE energies were also calculated.
The structures of dispersion-bound complexes were optimized at the DFT-D3 level, and harmonic Delta ZPVE energies were determined at the same level as well. For comparison, CCSD(T)/CBS D-0 energies were also evaluated for both types of complexes.
The CCSD(T)/ CBS interaction energy was constructed as the sum of MP2/CBS interaction energy, extrapolated from aug-cc-pVTZ and aug-cc-pVQZ basis sets, and DCCSD(T) correction, determined with the aug-cc-pVDZ basis set. The Delta ZPVE energies were determined for all complexes at the harmonic level and for selected complexes, these energies were also calculated using second-order vibration perturbation (VPT2) theory.
For H-bonded complexes, the harmonic CCSD(T)/CBS D-0 energies were in better agreement with the experimental values (with a mean relative error (MRE) of 6.2%) than the RI-MP2/cc-pVTZ D-0 (a MRE of 12.3%). The same trend was found for dispersion-bound complexes (6.2% (MRE) at CCSD(T)/CBS and 7.7% (MRE) at the DFT-D3 level).
When the anharmonic DZPVE term was included instead of harmonic one, the agreement between theoretical and experimental D-0 deteriorated for H-bonded as well as dispersion-bound complexes. Finally, the applicability of "diagonal approximation'' for determining the anharmonic Delta ZPVE was shown.
For the phenol center dot center dot center dot H2O complex, the Delta ZPVE energy calculated at the VPT2 level and on the basis of "diagonal approximation'' differed by less than 0.1 kcal mol(-1).