Investigating the Accuracy of Water Models through the Van Hove Correlation Function

Abstract

We present molecular-simulation-based calculations of the Van Hove correlation function (VHF) of water using multiple modeling approaches: classical molecular dynamics with simple three-site nonpolarizable models, with a polarizable model, and with a reactive force field; density functional tight-binding molecular dynamics; and ab initio molecular dynamics. Due to the many orders of magnitude difference in the computational cost of these approaches, we investigate how small and short the simulations can be while still yielding sufficiently accurate and interpretable results for the VHF. We investigate the accuracy of the different models by comparing them to recently published inelastic X-ray scattering measurements of the VHF. We find that all of the models exhibit qualitative agreement with the experiments, and in some models and for some properties, the agreement is quantitative. This work lays the foundation for future simulation approaches to calculating the VHF for aqueous solutions in bulk and under nanoconfinement.

Quantum chemical investigation of the effect of alkali metal ions on the dynamic structure of water in aqueous solutions

“…The rst 100 ps of the trajectory was used for equilibration, and the remaining 1.5 ns of the run was used for analysis, where MD trajectory snapshots were collected every 10 fs. We note that, based on the simulations with different system sizes (1000, 512, 250 and 128 water molecules) for bulk water, Matsumoto et al 44 found that no signicant system size effects are introduced in the smaller systems, and that 128 water molecules are sufficient to study the VHF and its peak behaviors with time. Also, the typical system size used in the study of aqueous ions with AIMD-based simulations is less than or equal to 128 molecules, with the length of the simulation within a few hundred ps.…”

“…The rst peak of the pure water VHF at t ¼ 0 is at z2.8 Å and the second peak is at z4.5 Å, consistent with the static PDF of pure water. As time elapses, the rst peak position moves towards higher values of r and the second peak position moves towards lower values of r. 44 As discussed above for the static PDFs, the addition of ions can signcantly inuence the water VHF and even introduce distinct new spectral features. Unless otherwise noted, we focus on a salt concentration of 1.2 mol kg À1 molality for clarity, and is sufficient to illustrate the important effects of the salt ions on the water VHF.…”

“…To this end, following the approach described in ref. 22,44, we have fitted A ( t ) ( t ≤ 0.75 ps) to a compressed exponential function of the form A ( t ) = a 1 e (− t / τ 1 ) γ 1 + a 2 e (− t / τ 2 ) γ 2 , where τ 1 and τ 2 represent the relaxation times of the first-step and second step-decay. The fitting results are listed in Table 3.…”

“…In that study, the DC-DFTB3-D3(BJ)/3ob method was found to reproduce the overall nature of the VHF and decay characteristics of various peaks in very good agreement with experiment. 44 For the purpose of this work, we rst compared the hydration shell structure of cations obtained from DFTB/MD simulations with the ab initio MD (AIMD) results reported by Wang et al 14 as shown in Fig. 1.…”

“…Further, the use of non-classical methods in the study of VHF is limited to pure water. 44 Investigations of the VHF of aqueous solution from molecular simulations have thus far been limited to classical MD simulations. 19,25 Here, we performed MD simulations at the DFTB level on the aqueous solutions of the chloride salts KCl, NaCl, and MgCl 2 .…”