Convergent Concordant Mode Approach for Molecular Vibrations: CMA-2

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Accurate calculation of molecular vibrational frequencies is crucial for understanding molecular properties and reactions.
• Existing computational methods face limitations in system size and theoretical rigor.
• The concordant mode approach (CMA) offers a promising strategy to overcome these limitations.

Purpose of the Study:

• To advance the concordant mode approach (CMA) hierarchy for molecular vibrational frequency computations.
• To benchmark CMA performance against high-level coupled cluster singles and doubles with perturbative triples (CCSD(T))/cc-pVTZ calculations.
• To develop efficient methods for achieving high accuracy in vibrational frequency calculations.

Main Methods:

• Utilized second-order Møller-Plesset perturbation theory (MP2)/cc-pVTZ for generating normal modes (Level B) within CMA.
• Developed a convergent CMA-2 method employing Hartree-Fock (HF) and MP2 or density functional theory (DFT) data.
• Introduced ξ parameters to select sparse off-diagonal force field elements for explicit evaluation at higher levels (Level A).

Main Results:

• CMA-0A with MP2/cc-pVTZ reproduced 1501 benchmark frequencies with a mean absolute error (MAE) of 0.11 cm^-1.
• CMA-2 achieved an average maximum absolute error of 0.17 cm^-1 with a modest cost increase (33%).
• The new CMA methods successfully computed frequencies for diverse vibrations in 1-(1H-pyrrol-3-yl)ethanol.

Conclusions:

• The enhanced CMA hierarchy significantly improves the accuracy and efficiency of molecular vibrational frequency calculations.
• MP2/cc-pVTZ is an excellent choice for generating Level B normal modes in CMA.
• The CMA-2 method provides a robust and computationally feasible pathway to high-accuracy vibrational frequencies.