Cascaded group-additivity ONIOM: A new method to approach CCSD(T)/CBS energies of large aliphatic hydrocarbons

We report a cascaded group-additivity (CGA) ONIOM method for high-level energy calculations of large aliphatic hydrocarbon molecules by combining the group additivity and two-layer ONIOM methods. This hybrid method is implemented by partitioning the target molecule into individual groups, which are cascaded via the overlapping between them. The energy of the entire molecule is first calculated at a low level of theory such as M06-2x/cc-pVTZ. Then all the groups and their overlappings are treated at the levels of CCSD(T)/CBS and M06-2x/cc-pVTZ to obtain their energy difference to be used as the energy correction. We selected small-to-middle size aliphatic hydrocarbons including 79 C₄–C₈ molecules as the validation set to demonstrate the feasibility of the CGA-ONIOM method, followed by the calculations of 12 representative C₁₀, C₁₂ and C₁₆ aliphatic hydrocarbons (including normal-, branched-, cyclo- and unsaturated categories). Our calculations agree well with the reference values available in the literature with the modest deviation around 1.0 kcal mol⁻¹. Compared with the conventional CCSD(T)/CBS calculation of the whole molecule, the computational cost can be dramatically reduced by a factor of ∼10² for molecules with 10 carbons and ∼10⁴ for molecules with 16 carbons. Considering its outstanding computational efficiency and accuracy, our proposed CGA-ONIOM method is promising for combustion chemistry studies of large fuel molecules at a high level of theory.