Approximate first-principles anharmonic calculations of polyatomic spectra using MP2 and B3LYP potentials:...

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Accurate prediction of molecular vibrational spectra is crucial for understanding molecular structure and dynamics.
• Anharmonic effects significantly influence vibrational frequencies and intensities, necessitating advanced computational methods.

Purpose of the Study:

• To evaluate the accuracy of MP2 and B3LYP potential energy surfaces for anharmonic vibrational spectroscopy calculations.
• To compare computed vibrational frequencies and intensities with experimental data for a series of molecules.

Main Methods:

• Anharmonic vibrational spectroscopy calculations were performed using the vibrational self-consistent field with second-order perturbation correction (VSCF-PT2) method.
• MP2 and B3LYP methods were employed to compute potential energy surfaces for molecules including HNO3, C2H4, C2H4O, H2SO4, CH3COOH, glycine, and alanine.

Main Results:

• Both MP2 and B3LYP methods yielded results in good agreement with experimental vibrational frequencies and intensities.
• MP2 showed slightly better agreement with experimental frequencies compared to B3LYP.
• Statistical analysis revealed predictable deviations: B3LYP favored red-shifted frequencies (-2%), while MP2 favored blue-shifted frequencies (+2%).
• Percentage deviations for intensities were larger than for frequencies, but still showed good accord with experiment.

Conclusions:

• MP2 and B3LYP potential energy surfaces, when used with VSCF-PT2 calculations, effectively capture anharmonic effects in molecular vibrational spectra.
• These computational approaches provide reliable predictions for molecular spectroscopy, aiding in structural and dynamic analysis.