Validating Molecular Dynamics and Joint DFT Predictions of Interfacial Water Using X-Ray Reflectivity

Capturing the structure and properties of water next to a solid surface presents a unique challenge for both Kohn-Sham and classical density-functional theories due to a complex interplay between chemical binding, electrostatic interactions, steric effects, and dispersion. X-ray reflectivity measurements determine the electron density of aqueous interfaces with high precision, but rely on model-dependent fitting to obtain the corresponding structural model. We present a validation protocol which enables calculation of interfacial X-ray structure factors from theory for direct comparison to experimental measurements. We apply this protocol to benchmark first principles molecular dynamics, classical molecular dynamics, and joint DFT simulations of the Al₂O₃/water interface, probing the effect of pressure, temperature, and finite size upon the predicted structural model. We explore the relative strengths and weaknesses of each class of theory, gaining insights into the bonding and structural properties of interfacial water which will aid future development of more accurate electronic and classical density-functionals.