Phase-Sensitive Vibrational SFG Spectra from Simple Classical Force Field Molecular Dynamics Simulations

We show that phase-sensitive vibrational sum frequency generation (SFG) spectra of solid/water and air/water interfaces, neutral and charged, can be successfully predicted using classical molecular dynamics (CMD) simulations in combination with simple nonpolarizable force fields (FFs). This can be achieved when employing velocity-velocity autocorrelation functions weighted by parameterized Raman and atomic polar tensors for the computation of the SFG. This procedure avoids computing polarizability tensors and dipole moments using either costly ab initio molecular dynamics (AIMD) simulations or CMD simulations with more complex and computationally demanding FFs. Such a methodology paves the way to a broad usage and computationally low-cost theoretical SFG spectroscopy, as even flexible nonpolarizable water models and common FFs for inorganic surfaces can provide good predictions of the SFG spectra, in rather good qualitative agreement with AIMD and/or experiments. The strongly reduced computational cost in our approach opens the possibility to study larger systems for long periods of time, for example, allowing a detailed characterization of the electric double-layer formation at interfaces with environmentally relevant ionic concentrations (mM), extracting fingerprints by theoretical CMD-SFG spectroscopy.