[Paper Review] The Boson peak in supercooled water

Pradeep Kumar, Kjartan Thor Wikfeldt+3 more|arXiv (Cornell University)|May 19, 2013

Material Dynamics and Properties54 references4 citations

TL;DR

This study uses molecular dynamics simulations of the TIP4P/2005 water model to show that the Boson peak in supercooled water emerges below the Widom line temperature $T_W \approx 230$ K, coinciding with a structural crossover to a low-density-like liquid. The peak arises from transverse acoustic phonons in the parent ice Ih crystal, indicating the Boson peak is a general feature of tetrahedral liquids undergoing a liquid-liquid phase transition, not exclusive to confinement.

ABSTRACT

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line $T_W$. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

Motivation & Objective

• To investigate the origin of the experimentally observed Boson peak in supercooled water under nanoconfinement and in hydration layers.
• To determine whether the Boson peak in supercooled water is an artifact of confinement or a bulk property linked to structural and dynamic crossovers.
• To explore the connection between the Boson peak and the vibrational modes of the parent crystal, ice Ih.
• To test whether the Boson peak in water is related to the liquid-liquid phase transition and the Widom line.

Proposed method

• Molecular dynamics simulations of the TIP4P/2005 water model at atmospheric pressure across temperatures from 210 K to 260 K, spanning the Widom line at ~230 K.
• Use of NPT and NVT ensembles for equilibration, followed by NVE integration for dynamical property calculation with 0.2–1.0 fs time steps.
• Calculation of the incoherent dynamic structure factor $S_S(k,\omega)$ and reduced vibrational density of states (VDOS) to identify the Boson peak.
• Analysis of transverse and longitudinal correlation functions to assess phonon contributions to the Boson peak intensity.
• Quenching of equilibrium configurations to inherent structures and diagonalization of the Hessian matrix to compute vibrational eigenmodes and participation ratios.
• Validation using the TIP5P water model across 240–270 K to confirm robustness of results across different water models.

Experimental results

Research questions

• RQ1 Does the Boson peak in supercooled water originate from confinement effects or from intrinsic bulk properties?
• RQ2 Is the onset of the Boson peak correlated with the Widom line and the structural crossover to low-density-like liquid?
• RQ3 Are the vibrational modes responsible for the Boson peak related to transverse acoustic phonons in the parent ice Ih crystal?
• RQ4 Does the frequency of the Boson peak in bulk supercooled water match experimental observations in nanoconfined water?
• RQ5 Is the Boson peak in water a general feature of tetrahedral liquids undergoing a liquid-liquid phase transition?

Key findings

• The Boson peak in supercooled bulk water emerges precisely at the Widom line temperature $T_W \approx 230$ K, coinciding with a structural crossover to a low-density-like liquid.
• The frequency of the Boson peak in TIP4P/2005 water reaches $\omega \approx 45$ cm$^{-1}$ in the low-temperature regime, matching experimental values in nanoconfined water.
• The Boson peak is primarily driven by transverse acoustic phonons in the parent ice Ih crystal, as confirmed by correlation function analysis and mode decomposition.
• The peak frequency in the LDA glass phase approaches $45$ cm$^{-1}$, aligning with the transverse acoustic peak in hexagonal ice simulations and experiments.
• Finite-size effects are minimal, as simulations with up to $N = 45,000$ molecules show consistent results across system sizes.
• The TIP5P water model also exhibits a Boson peak below its Widom line, confirming the phenomenon is robust across water models and not model-specific.