Popular theoretical methods predict benzene and arenes to be nonplanar

Area of Science:

• Computational chemistry
• Quantum chemistry
• Molecular modeling

Background:

• Planar benzene is a fundamental aromatic molecule.
• Accurate prediction of molecular geometry is essential in chemistry.
• Previous studies have shown discrepancies in benzene's predicted structure.

Purpose of the Study:

• To investigate the computational failure to predict planar benzene as a minimum.
• To identify the cause of spurious imaginary frequencies in benzene and related molecules.
• To determine the necessary computational conditions for accurate predictions of arene planarity.

Main Methods:

• Utilized second-order Møller–Plesset perturbation theory (MP2) with various basis sets.
• Analyzed vibrational frequencies to identify imaginary modes indicating instability.
• Examined the role of basis set incompleteness error (BSIE) in computational predictions.

Main Results:

• MP2 calculations with unbalanced Pople-type basis sets incorrectly predict planar benzene.
• Spurious imaginary frequencies correspond to out-of-plane distortions (chair and boat conformations).
• This failure is observed in other pi-delocalized systems like pyridine and naphthalene.

Conclusions:

• An unbalanced basis set incompleteness error (BSIE) is responsible for the inaccurate prediction of planar benzene.
• Correlated wave function methods require balanced, correlation-consistent basis sets for reliable arene geometry predictions.
• The findings highlight the importance of basis set selection in computational chemistry.