Connectivity-Based Hierarchy for theoretical thermochemistry: assessment using wave function-based methods.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Thermochemistry

Background:

• The Connectivity-Based Hierarchy (CBH) is a generalized method for predicting thermochemical properties of organic molecules.
• Previous evaluations used density functional theory (DFT).

Purpose of the Study:

• To perform a wave function-based analysis of the CBH method.
• To investigate the influence of electron correlation effects on reaction energies and enthalpies of formation.
• To assess the performance of CBH for strained and unstrained molecules.

Main Methods:

• Wave function-based analysis using Hartree-Fock (HF), Møller–Plesset perturbation theory (MP2), and coupled cluster with singles and doubles and perturbative triples (CCSD(T)) levels of theory.
• Evaluation using polarized double-ζ and triple-ζ basis sets.
• Analysis of reaction energies and enthalpies of formation.

Main Results:

• For unstrained molecules, HF, MP2, and CCSD(T) yield accurate enthalpies of formation with modest basis sets.
• For strained molecules, correlated schemes (MP2, CCSD(T)) provide accurate enthalpies of formation, though reaction energies are not small.
• The MP2 method at the CBH-2 rung shows performance comparable to the CCSD(T) method at the CBH-3 rung for large nonaromatic molecules.

Conclusions:

• Small reaction energies are not a reliable sole criterion for evaluating thermochemical schemes, especially for molecules with strain or aromaticity.
• Wave function methods confirm the accuracy of the CBH approach for predicting thermochemical properties.
• The CBH method, particularly with MP2, offers a computationally efficient yet accurate approach for large organic molecules.