Toward Accurate Quantum Mechanical Thermochemistry: (2) Optimal Methods for Enthalpy Calculations from Comprehensive...

Area of Science:

• Computational Chemistry
• Thermochemistry
• Physical Organic Chemistry

Background:

• Accurate standard enthalpies of formation (ΔHf°) are vital for chemical engineering and scientific applications.
• Previous work established a foundation for benchmarking computational methods.
• A systematic evaluation of numerous model chemistries is needed for reliable ΔHf° predictions.

Purpose of the Study:

• To systematically benchmark 284 model chemistries for computing standard enthalpies of formation (ΔHf°) of small organic molecules.
• To derive and assess Petersson- and Melius-type bond-additivity corrections (BACs) for enhancing accuracy.
• To provide practical guidance for selecting cost-effective and accurate computational methods.

Main Methods:

• Benchmarking of 284 model chemistries including semiempirical, DFT, wave function theory, and composite schemes.
• Derivation of Petersson- and Melius-type bond-additivity corrections (BACs) using a database of 421 reference species.
• Validation of top-performing methods on an independent test set of 500 species (ions, radicals, etc.).

Main Results:

• Bond-additivity corrections (BACs) substantially improve accuracy for most methods and species, particularly neutral singlets.
• Composite schemes combining DFT geometries with local coupled-cluster energies achieve high accuracy (near 1 kcal/mol).
• The DLPNO-CCSD(T)-F12d/cc-pVTZ-F12//ωB97X-D/def2-TZVPD method with Petersson BAC achieved the best MAE of 0.57 kcal/mol.
• A computationally cheaper variant (DLPNO-CCSD(T)-F12d/cc-pVDZ-F12//GFN2-xTB) offered a good balance of cost and accuracy (MAE 0.96 kcal/mol).

Conclusions:

• BACs are crucial for accurate ΔHf° calculations, especially for neutral organic molecules.
• Carefully selected composite methods provide excellent accuracy and cost-efficiency.
• Further development of high-accuracy thermochemical datasets is needed for charged and open-shell species.
• This benchmark guides the selection of optimal computational strategies for large-scale thermochemical calculations.