Using molecular dynamics simulations of UO2-a type II superionic conductor-we identify a well-defined onset of dynamic disorder (Tα), which is remarkably correlated to a nontrivial advance of dynamical heterogeneity (DH). Quantified by the correlations in the dynamic propensity and van Hove self-correlation function, the DH is shown to grow with increasing temperature from Tα, peak at an intermediate temperature between Tα and Tλ-the superionic transition temperature-and then recede. Surprisingly, the DH attributes are not uniform across the temperatures-our investigation shows a low temperature (αT) stage DH, which is characterized by weak correlations and a plateaulike period in the correlations of the propensity, and a high temperature (λT) stage DH with strong correlations that are analogous to those in typical supercooled liquids. Our work, which has rigorously identified the onset of superionicity, gives a different direction for interpreting scattering experiments on the basis of statistical, correlated dynamics.

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Water is fundamental to the biochemistry of enzymes. It is well known that without a minimum amount of water, enzymes are not biologically active. Bare minimal solvation for biological function corresponds to about a single layer of water covering enzymes' surfaces. Many contradictory studies on protein-hydration-water-coupled dynamics have been published in recent decades. Following prevailing wisdom, a dynamical crossover in hydration water (at around 220 K for hydrated lysozymes) can trigger larger-amplitude motions of the protein, activating, in turn, biological functions. Here, we present a molecular-dynamics-simulation study on a solvated model protein (hen egg-white lysozyme), in which we determine, inter alia, the relaxation dynamics of the hydrogen-bond network between the protein and its hydration water molecules on a residue-per-residue basis. Hydrogen-bond breakage/formation kinetics is rather heterogeneous in temperature dependence (due to the heterogeneity of the free-energy surface), and is driven by the magnitude of thermal motions of various different protein residues which provide enough thermal energy to overcome energy barriers to rupture their respective hydrogen bonds with water. In particular, arginine residues exhibit the highest number of such hydrogen bonds at low temperatures, losing almost completely such bonding above 230 K. This suggests that hydration water's dynamical crossover, observed experimentally for hydrated lysozymes at ∼220 K, lies not at the origin of the protein residues' larger-amplitude motions, but rather arises as a consequence thereof. This highlights the need for new experimental investigations, and new interpretations to link protein dynamics to functions, in the context of key interrelationships with the solvation layer.

Elucidating freezing mechanisms of liquid water into ice, especially in "No Man's Land" (150 K< T < 235 K), carries scientific and technological importance. Indeed, superior predictions of upper-troposphere cirrus-cloud formation and surface-bound ice-fog formation constitute powerful motivations in addition to unravelling long-standing puzzles such as persistent liquid fogs well below frost point and understanding interstellar-space water states, together with advancing cryopreservation technology. Unlocking the secrets of water's anomalous deep-cooling complexities, such as structural ordering and microscopic nucleation mechanisms, are the subject of lively debate. Exploring nucleation mechanism in No Man's Land (NML) is technically demanding, owing to rapid nucleation rates with, unsurprisingly, very few reported experimental studies. However, amorphization is a key intermediate stage in NML-based nucleation, and it is also not particularly well understood. In this microsecond long molecular dynamics study, we have explored microstructural processes involved in the amorphization of aggressively quenched supercooled water nanodroplets in the gas phase where surface effects are non-negligible. A dynamically arrested state is observed in these droplets that resembles structurally low-density amorphous polymorphs of ice. Importantly, the curved geometry of the nanodroplets themselves is seen to inhibit amorphization relative to bulk systems under identical thermodynamic conditions.

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare the relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al (2006 Phys. Rev. B 73 195123) and compare the effectiveness of the correction scheme for Al and the 3d transition metals.

Elucidating water-to-ice freezing, especially in "No Man's Land" (150 K < T < 235 K), is fundamentally important (e.g., predicting upper-troposphere cirrus-cloud formation) - and elusive. An oft-neglected aspect of tropospheric ice-crystallite formation lies in inevitably-present electric fields' role. Exploring nucleation in No Man's Land is technically demanding, owing to rapid nucleation rates, to mention nothing of difficulties of applying relevant electric fields thereto. Here, we tackle these intriguing open questions, via non-equilibrium molecular-dynamics simulations of sub-microsecond formation of rhombus-shaped ice I nano-crystallites from aggressively-quenched supercooled water nano-droplets in the gas phase, in external static electric fields. We explore droplets' nano-confined geometries and the entropic-ordering agent of external electric fields as a means of realising cubic-ice formation, especially with very few stacking faults and defects.

Elucidation of the role of hydration water underpinning dynamical crossover in proteins has proven challenging. Indeed, many contradictory findings in the literature seek to establish either causal or correlative links between water and protein behavior. Here, via molecular dynamics, we compute the temperature dependence of mean-square displacement and translational self-diffusivities for both hen egg white lysozyme and its hydration layer from 190 to 300 K. We find that the protein's mobility increases sharply at ∼230 K, indicating dynamical onset; concerted motion with hydration-water molecules is evident up to ∼285 K, confirming dynamical correlation between them. Exploring underlying mechanisms of such concerted motion, we scrutinize the water-protein hydrogen-bonding network as a function of temperature, noting sharp deviation from linearity of the hydrogen bond number's profile with temperature originating near the protein dynamical transition. Our studies reveal a common temperature profile/dependence of self-diffusivity values of the protein, hydration water, and the bulk solvent, originating from a common dependence on the bulk solvent viscosity, η. The key mechanistic role adopted by the protein-water hydrogen bond network in relation to the onset of proteins' dynamical transition is also discussed.

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