Approximating large-basis coupled-cluster theory vibrational frequencies using focal-point approximations

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• High-accuracy quantum chemistry computations are often computationally expensive.
• Accurate prediction of molecular vibrational frequencies is crucial for chemical analysis.
• Existing methods like coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] provide high accuracy but are time-consuming.

Purpose of the Study:

• To evaluate the performance of focal-point approximation methods for calculating molecular frequencies.
• To compare the accuracy and computational cost of focal-point methods against standard high-level computations.
• To assess the ability of focal-point methods to approach the complete basis set (CBS) limit efficiently.

Main Methods:

• Employed focal-point approximation by combining second-order Møller-Plesset perturbation theory (MP2) with CCSD(T).
• Calculated harmonic and fundamental vibrational frequencies using second-order vibrational perturbation theory (VPT2).
• Compared focal-point CCSD(T) results with experimental data for 20 small molecules (up to six atoms).

Main Results:

• Focal-point CCSD(T) with triple-ζ basis sets achieved accuracy comparable to standard CCSD(T) extrapolated to the complete basis set (CBS) limit using larger basis sets.
• The focal-point method yielded a mean absolute error of only 7.3 cm⁻¹ for fundamental frequencies compared to experimental values.
• For water (H2O), the focal-point approach required only 3% of the computational time of extrapolated CCSD(T), with greater savings expected for larger molecules.

Conclusions:

• The focal-point approximation offers a computationally efficient route to high-accuracy molecular frequency predictions.
• This method provides a practical alternative to standard, resource-intensive high-level quantum chemistry calculations.
• The cost-effectiveness of focal-point methods is expected to increase significantly with molecular size.