This work focuses on developing a novel computational technique to study molecular vibrations through the Raman scattering Driven Molecular Dynamics (DMD) method. While the main criterion for IR absorption is a net change in the dipole moment in a molecule as it vibrates, presently we wish to predict and analyze vibrational spectra to study symmetric vibrational modes that are IR inactive or weakly active while strongly Raman active. A newly developed method was tested on CO2, H2O, CH4, and C20 molecules. The molecular structures were optimized, vibrational frequencies, IR, and Raman intensities calculated using the normal mode analysis. We used MP2/cc-pVDZ (CO2, H2O, CH4) and DFT-B3LYP/6-31G* (C20) computational levels to generate the potential energy surfaces (PES). Modified Hamilton’s equations of motion include a driven term with polarizability tensor surface (PTS), the function that couples the incoming and scattered lights. The resonant motions at fundamental frequencies were analyzed and described in terms of atomic displacements and absorbed energy profiles. We show that it is possible to interpret experimental infrared and Raman spectra through a combination of ab initio molecular dynamics simulations and normal mode analysis. Our results show that DMD simulations provide key probes in the chemical identification of molecules. The DMD method can detect symmetric stretch vibrations that are not accessible in the IR spectroscopy. Future prospects of this method include exploration of the role of anharmonic effects.

Driven molecular dynamics simulations of Raman spectra: Vibrations of symmetric molecules

ACS Fall 2021

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