Simplified Ab Initio Molecular Dynamics-Based Raman Spectral Simulations

Abstract

We describe a simplified approach to simulating Raman spectra from ab initio molecular dynamics (AIMD) calculations. The protocol relies on on-the-fly calculations of approximate molecular polarizabilities using the well-known sum over orbitals (as opposed to states) method. This approach bypasses the more accurate but computationally expensive approach to calculating molecular polarizabilities along AIMD trajectories, i.e., solving the coupled perturbed Hartree–Fock/Kohn–Sham equations. We demonstrate the advantages and limitations of our method through a few case studies targeting molecular systems of interest to surface- and/or tip-enhanced Raman spectroscopy practitioners.

Power-Dependent Dual Analyte Tip-Enhanced Raman Spectral Imaging

“…This image is traced at 2225 cm –1 , which corresponds to the CN stretching vibrational resonance of TBN . Images taken at ∼1335 cm –1 , the predominant Raman-active vibration of NTP, are very similar to those of their analogue in Figure A (see Figure S1). As discussed in prior works, , these images are mostly governed by local optical fields in the immediate vicinity of the Au microplate.…”

“…A TERS map of a triangular microplate coated with both TBN and NTP is shown in Figure A. This image is traced at 2225 cm –1 , which corresponds to the CN stretching vibrational resonance of TBN . Images taken at ∼1335 cm –1 , the predominant Raman-active vibration of NTP, are very similar to those of their analogue in Figure A (see Figure S1).…”

“…Two spectrally distinct resonances at 1335/2225 cm –1 corresponding to the NO/CN stretching vibrations of NTP/TBN may be used to track the two analytes in the frequency domain, even though the TERS maps in both cases are dominated by the structure of the local optical fields (see Figure A and Figure S1). On the basis of prior density functional theory simulations of the Raman spectra of both molecules, we can state that the relative scattering cross sections of NTP/TBN at 1335/2225 cm –1 are of the same order of magnitude; this is at least the case for the bare molecules in the gas phase. Our observation of brighter NTP signatures in our measurements indicates that either (i) chemical enhancement is larger for chemisorbed NTP or (ii) NTP molecules are more prevalent along the edges of the plate.…”

“…The spectra themselves contain the signatures of both analytes (see Figure B and C). Two spectrally distinct resonances at 1335/2225 cm –1 corresponding to the NO/CN stretching vibrations of NTP/TBN may be used to track the two analytes in the frequency domain, even though the TERS maps in both cases are dominated by the structure of the local optical fields (see Figure A and Figure S1). On the basis of prior density functional theory simulations of the Raman spectra of both molecules, we can state that the relative scattering cross sections of NTP/TBN at 1335/2225 cm –1 are of the same order of magnitude; this is at least the case for the bare molecules in the gas phase.…”

High spatial resolution ambient tip-enhanced (multipolar) Raman scattering

“…Both spectra contain the 3 familiar Raman-active modes (highlighted using red rectangles) that derive their intensities through a. 20,26 The blue trace contains an additional dominant mode (highlighted using a blue rectangle) and otherwise closely resembles the G 2 spectrum that is dominated by the same resonance. The black experimental trace is distinct, containing three peaks that all derive their intensities from A 2 .…”