Binding Energies of Hydrogen-Bonded Complexes

To calculate an accurate binding energy for an intermolecular hydrogen-bonded complex, you can use the Hydrogen Bond workflow, which is encoded in the script hydrogen_bond.py (see hydrogen_bond.py: Binding Energy of a Hydrogen-Bonded Complex for instructions) with a Maestro interface. In this workflow, the binding energy is calculated using two correlation-consistent basis sets (cc-pVTZ(-f) and cc-pVQZ(-g)) and LMP2 theory, including corrections for basis set superposition error. The final correction to the binding energy uses two parameters from a fit of the energies to reference energies that were obtained from CCSD(T) calculations extrapolated to the basis set limit. The training set consisted of five small molecule complexes, described in Ref. [28]. The binding energies are accurate to within 0.5 kcal/mol when compared to the extrapolated CCSD(T) energies.

The protocol is based on that described in Ref. [28], except that all geometry optimizations are carried out using X3LYP/6-31G** instead of LMP2/cc-pVTZ(-f). The X3LYP density functional gives very good geometries for hydrogen-bonded complexes and is much faster than LMP2. The energy calculations, however, are still carried out using LMP2 with the cc‑pVTZ(‑f) and cc-pVQZ(-g) basis sets. No CCSD(T) calculations are performed in the workflow.

An option is provided to use the DFT energies rather than the LMP2 energies. In a test for a series of ten hydrogen-bonded complexes, the RMS error using the DFT energies was 0.72 kcal/mol, while the RMS error using the default LMP2 method was 0.34 kcal/mol. Other options are provided to control the optimization process.

To run a hydrogen bond binding energy calculation, select the desired structures in the Project Table or Entry List, click the Tasks button and browse to Quantum Mechanics → Hydrogen Bond Strength. For information on the panel, see HBond Panel.

When the job finishes, the optimized structure of the complex is incorporated into the project, with the binding energy, in kcal/mol, as a property.