Anharmonic Vibrational Frequency Calculations Are Not Worthwhile for Small Basis Sets.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Spectroscopy

Background:

• Anharmonic calculations offer high accuracy with advanced methods.
• The performance of anharmonic calculations with economical methods is less understood.
• Scaling is a common practice to correct harmonic frequency predictions.

Purpose of the Study:

• To evaluate the accuracy of anharmonic and harmonic vibrational frequency predictions.
• To compare these predictions using popular, economical electronic structure methods (Hartree-Fock, MP2, DFT) and basis sets (6-31G(d), 6-31+G(d,p)).
• To assess if scaled anharmonic calculations outperform scaled harmonic calculations.

Main Methods:

• Vibrational perturbation theory for anharmonic calculations.
• Hartree-Fock, second-order perturbation theory (MP2), and density functional theory (DFT) for electronic structure.
• Utilized data from the NIST Computational Chemistry Comparison and Benchmark Database (CCCBDB).

Main Results:

• Anharmonic frequencies were closer to experimental fundamentals than harmonic frequencies.
• Scaled anharmonic calculations did not show superior accuracy compared to scaled harmonic calculations for the tested methods and basis sets.
• The study analyzed 3939 vibrations across 358 molecules.

Conclusions:

• For economical electronic structure methods and basis sets, scaled harmonic calculations are as accurate as scaled anharmonic calculations.
• The presumed advantage of anharmonic calculations may not materialize with commonly used, less computationally intensive methods.
• Further investigation is needed for high-level methods to confirm anharmonic benefits.