Interactions between large molecules pose a puzzle for reference quantum mechanical methods.

Area of Science:

• Computational chemistry
• Quantum mechanics
• Molecular interactions

Background:

• Quantum-mechanical methods like diffusion Monte Carlo (DMC) and coupled cluster [CCSD(T)] are vital for understanding molecular interactions.
• These methods provide accurate interaction energies for small organic molecules, serving as benchmarks for other computational approaches.

Purpose of the Study:

• To assess the agreement between CCSD(T) and DMC methods for interaction energies in larger, polarizable supramolecular systems.
• To identify limitations in current benchmark quantum chemical methods for extended molecular systems.

Main Methods:

• Utilized diffusion Monte Carlo (DMC) and coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] quantum-mechanical methods.
• Calculated interaction energies for a set of polarizable supramolecular complexes.

Main Results:

• Inconsistencies were observed between CCSD(T) and DMC interaction energies for the studied supramolecular systems.
• Disagreements of up to 8 kcal mol⁻¹ were found in key systems, indicating method-dependent variations.

Conclusions:

• The benchmark accuracy of CCSD(T) and DMC methods is not consistently achieved for extended molecular systems.
• Increased caution is necessary when determining reproducible non-covalent interactions in larger, complex molecular assemblies.