Potential energy surfaces and dynamic properties via ab initio composite and density functional approaches

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Theoretical Chemistry

Background:

• Vibrational spectroscopy provides crucial molecular insights.
• Current methods often use the harmonic approximation, which fails for anharmonic systems.
• Anharmonic potential energy surfaces offer an alternative for accurate frequency calculations.

Purpose of the Study:

• To evaluate density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) for generating anharmonic frequencies.
• To investigate the impact of functional choice, basis set, and mode-coupling on accuracy.
• To develop and test a multilevel DFT:ccCA approach for improved vibrational frequency prediction.

Main Methods:

• Calculation of anharmonic frequencies using vibrational self-consistent field (VSCF) and post-VSCF methods.
• Generation of potential energy surfaces using DFT and ccCA.
• Development of a multilevel DFT:ccCA approach combining strengths of both methods.
• Comparison of computed frequencies with experimental data.

Main Results:

• ccCA potentials yielded lower absolute deviations than DFT potentials.
• DFT better described bending modes compared to ccCA.
• The multilevel DFT:ccCA approach, using VCIPSI-PT2, produced frequencies closer to experimental values than scaled harmonic frequencies.
• Post-VSCF methods enhance the accuracy of computed infrared spectra.

Conclusions:

• A multilevel DFT:ccCA approach is effective for calculating anharmonic vibrational frequencies in larger molecules.
• This method offers a more accurate representation of computed infrared spectra compared to traditional harmonic approximations.
• The study highlights the importance of anharmonicity in molecular vibrational analysis.