Infrared intensities of : a true challenge for DFT methods

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate calculation of molecular properties is crucial in chemistry.
• Infrared (IR) intensities are important spectroscopic parameters.
• Theoretical methods are often used to predict IR intensities.

Purpose of the Study:

• To evaluate absolute infrared intensities of a specific molecule using diverse computational methods.
• To assess the agreement between different density functional theory (DFT) and ab initio methods.
• To identify the most reliable theoretical approach for calculating IR intensities.

Main Methods:

• Density Functional Theory (DFT) calculations
• Ab initio methods (e.g., QCISD, CCSD)
• Various basis sets
• Analysis of geometrical parameters and net atomic charges

Main Results:

• Significant disagreement in calculated infrared intensities across different theoretical levels.
• Qualitative differences (weak/strong) observed in calculated intensities.
• Geometrical parameters and net atomic charges showed less variation than intensities.

Conclusions:

• Current DFT and ab initio methods show considerable divergence in predicting IR intensities.
• The lack of experimental data hinders the selection of the best theoretical method.
• Development of new, more consistent computational methods is warranted.