Calculation of absolute Raman scattering cross-sections using vibrational self-consistent field/vibrational
Jhonatas R Carvalho1, Luciano N Vidal2
1Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas, USA.
Journal of Computational Chemistry
|June 22, 2022
Summary
This study calculates Raman scattering properties for small molecules like water and methane using advanced computational methods. Surprisingly, simpler approximations often matched experimental data better than complex ones for scattering cross-sections.
Area of Science:
- Computational Chemistry
- Molecular Spectroscopy
- Quantum Mechanics
Background:
- Accurate prediction of molecular properties is crucial for understanding chemical phenomena.
- Raman spectroscopy provides insights into molecular vibrations and structures.
- Advanced computational methods are needed to model complex molecular interactions.
Purpose of the Study:
- To calculate differential scattering cross-sections, depolarization ratios, and Raman shifts for small molecules.
- To compare the accuracy of different theoretical methods, including vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI).
- To investigate the influence of theoretical approximations on the prediction of Raman scattering properties.
Main Methods:
- Utilized configuration iteration wave functions of vibrational self-consistent field (VSCF) states.
- Employed the Placzek approximation for transition polarizabilities, focusing on electric dipole mechanisms.
- Calculated potential energy and electronic polarizability surfaces using CCSD(T) and CC3 methods with extensive basis sets.
- Included the vibrational angular momentum contribution of the Watson kinetic energy operator in the vibrational Hamiltonian.
Main Results:
- Raman transition wavenumbers were significantly improved by VSCF and VCI methods compared to harmonic predictions.
- Surprisingly, harmonic approximation and VSCF methods yielded scattering cross-sections in better agreement with experimental values than VCI.
- Depolarization ratios showed similar accuracy for H2O and C2D2 between harmonic and VCI methods, with VCI being more accurate for C2H2 and C2HD.
Conclusions:
- VSCF and VCI methods offer improved Raman wavenumbers over harmonic predictions.
- Discrepancies in scattering cross-sections suggest potential issues with experimental data or the description of electronic polarizability surfaces in VCI.
- The choice of theoretical method impacts the accuracy of predicted Raman scattering properties, necessitating careful consideration for specific molecular systems.
Keywords:
Raman spectrumacetyleneanharmonic wave functionscoupled-cluster electronic structuremethanewaterMore Related Videos
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