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Although CMD simulations produce qualitative agreement with the IXS response, the chosen models lack the details required to quantitatively reproduce the interactions of water. To do so, a multiscale approach with the inclusion of quantum mechanical and reactive approaches is required. Ab initio MD (AIMD) is a quantum mechanical approach that computes electronic structure calculations on the fly to generate forces for dynamics. Though computationally expensive, these calculations allow these methods to be unbiased to a given state or set of reference data. Density-functional tight-binding (DFTB) is an approximation to density functional theory that allows for routine larger scale simulations on nanosecond time scales. Reactive MD includes additional interaction terms to account for chemical reactions and hydrogen bonding. Here we present VHF results aimed at better understanding the robustness of molecular simulation techniques describing water at a multitude of time and length scales. Encompassed in this study are a range of classical, polarizable, and reactive force fields, as well as AIMD, and DFTB.
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