Ab initio molecular dynamics (MD) simulations represent a versatile approach to investigate the structure and dynamics of water interfaces, especially in the cases of heterogeneous systems where less computationally demanding techniques, such as force field approaches are not easily transferable.
We present here some recent results on the calculation of vibrational Sum Frequency Generation spectra, which employ suitable velocity-velocity correlation functions incorporating the appropriate selection rules [1,2].
We also present a new approach to investigate vibrational energy relaxation at water interfaces from the analysis of ab initio non-equilibrium MD trajectories. We follow the energy relaxation from a locally excited vibrational state using suitable descriptors based on vibrational density of states.
Our method is tested on different water systems, including bulk water and fluorite/water interfaces at different pH and can be used to provide a molecular interpretation of 2D-IR or 2D-SFG spectra. In the case of the water/fluorite interface at low pH we find that water behaves similarly to bulk water, while, in the case of high pH, instead, the energy relaxation is much slower. A molecular interpretation of the different time scales is provided.

[1] R Khatib, EHG Backus, M Bonn, MJ Perez-Haro, MP Gaigeot, M Sulpizi, Scientific Reports, 6 (2016)
[2] R. Khatib and M Sulpizi, J. Phys. Chem. Letters, 8 (6), 1310 (2017).