Dynamical Accuracy of Water Models on Supercooling

Abstract

Molecular dynamics (MD) simulations are commonly used to explore the structural and dynamical properties of supercooled bulk water in the so-called "no man's land" (NML) (150−227 K), where crystallization occurs almost instantaneously. This approach has provided significant insight into experimentally inaccessible phenomena. In this paper, we compare the dynamics of simulations using one-, three-, and four-body water models to experimentally measured quasielastic neutron scattering spectra. We show that the agreement between simulated and experimental data becomes substantially worse with a decrease in temperature toward the deeply supercooled regime. It was found that it is mainly the nature of the local dynamics that is poorly reproduced, as opposed to the macroscopic properties such as the diffusion coefficient. This strongly implies that the molecular mechanism describing the water dynamics is poorly captured in the MD models, and simulated structural and dynamical properties of supercooled water in NML must be interpreted with care.

Revisiting the modeling of quasielastic neutron scattering from bulk water

“…We observe that for Q > 1Å −1 the QENS data show asymptotic behavior, characteristic of jump diffusion of the atoms, while the MD simulations shows a linear growth indicating free diffusion with D MD t ≈ 3 • 10 −9 m 2 s . As previously reported [6,34], classic MD simulations have difficulties capturing the quantum effects of jump diffusion, making it plausible that the MD simulations continue probing free diffusion. This is somehow expected since the semi-classical approx- imation of the detailed balance relation, used to make the classical time correlation function equal to the quantum time correlation function, is only valid to the first order of ℏ meaning that for high Q-values the approximation breaks down.…”

Water Dynamics in Cancer Cells: Lessons from Quasielastic Neutron Scattering

“…Typically, there is a symbiotic relationship between QENS and molecular dynamics simulations, in which the latter can be used to interpret QENS results, and conversely, QENS can be used to benchmark and improve the potentials used in the simulations. This area of research has grown considerably in the last few years [ 31 , 32 , 33 , 34 ]. As well described by Zaccai [ 30 ], there is a reasonable perspective that this QENS/molecular dynamics interplay will soon be extended to the complex machinery of a living eukaryotic cell.…”

Uncovering the Dynamics of Confined Water Using Neutron Scattering: Perspectives

“…Analysis, interpretation, and modeling of QENS data is however complex, and this is a significant barrier to generating scientific innovation from the experiments performed. While QENS spectra contain the desired information to understand the dynamics in complex soft systems where long-range order is mostly absent [21], our current methods of analysis fail to completely capture the full dynamic nature arising from the multiscale atomic motions [22]. This hinders a more generalized understanding of the mechanisms behind their organizing nature.…”