We propose a general procedure to treat the model system boundary region within the ONIOM framework involving covalent double bonds. The basis of the treatment is to saturate the two-dangling valences of the covalently double bonded support atom with two hydrogen atoms to perform separate ONIOM calculations. We have developed a systematic procedure to project back the link-atom force components onto the host- and support-atoms of the real system. We have benchmarked our ONIOM model by evaluating the dissociation energies and atomic forces of 10 molecules with isolated and conjugated double bonds. ONIOM reaction energy shows a fair agreement with the full calculation (FC) at high-level, and the typical errors in reaction energies are within 1 kcal/mol. For benchmarking the accuracy of the forces, we have performed the geometry optimization, and the geometrical parameters show a fair agreement with the FC. The agreement of the ONIOM results with the FC shows that the adopted procedure is a pragmatic method to construct the model system for systems involving double bonds at the boundary region. The proposed procedure to treat link-atom is economical and adaptable within any fragment-based methods involving the construction of model systems, for the accurate estimation of energies and forces of large molecules.
ONIOM method, ab initio calculations, Hartree-Fock (HF) theory, Density Functional Theory (DFT), large molecules