Self-aggregation of water vapour molecules under external electric fields is systemically investigated by using molecular dynamics simulations. It is found that small water clusters aggregate into one-dimensional water nanowires along the electric field direction. The electric field strength plays a crucial role in tuning the nanowire structure. Under relatively weak electric fields such as E = 0.1 V Å⁻¹, square and pentagonal prism-like structures are preferred; when intermediate strength electric fields are applied (E = 1.0 V Å⁻¹), water nanowires featuring a disordered mixture of four-, five- and six-membered rings are formed; and an open ordered structure which is reminiscent of two-dimensional (2D) ice is observed when the field strength becomes very high (E > 3.0 V Å⁻¹). Bond parameter analysis based on density-functional theory calculations shows that the electric field affects anisotropically the conformation of water molecules as well as the hydrogen-bond properties. Along the electric field, the H–O bond is stretched and the hydrogen bond shrinks with field strength in contrast to the changes perpendicular to the electric field. As a result, the hydrogen bonding is enhanced along the electric field. Under very high electric fields, the anisotropic hydrogen-bond network opens up via breaking of the bonds perpendicular to the electric field and ultimately relaxes into a loose quasi-2D ordered network.

Understanding the structure–property relationships of energetic compounds is challenging. Herein, by including the experimental data, we systematically evaluated the microscopic characteristics of a series of transition metal carbohydrazide perchlorate (TMCP) complexes (MnCP, FeCP, CoCP, NiCP, ZnCP, and CdCP) by first-principles calculations. The calculated properties, i.e., lattice enthalpy, bulk modulus, and electronic structures, were correlated with their thermal decomposition temperatures and impact sensitivities, which indicated that the stability and sensitivity of the TMCP complexes were greatly changed through coordination with different metal ions. The trend was that a large lattice enthalpy indicated a better thermal stability. Complexes with a high impact sensitivity tended to have a smaller bulk modulus and pseudo-gap. The ultra-high impact sensitivity of FeCP may have been related to the unstable spin state with respect to the volume change in the lattice. The calculated bond order and bond dissociation energy did not fully reflect the impact and friction sensitivities in this study. In addition, the combination of crystal properties and local bond information may better describe the sensitivity trend for the TMCP energetic compounds. This analysis can be applied to other energetic compounds and may provide clues for the synthesis and assessment of novel energetic compounds.

The rotational dynamics of water near the ice/water interface and its relation with the crystal growth of ice are investigated by using molecular dynamics simulations. We find that the dipole-moment profiles of water adjacent to interfaces display an oscillation behavior, which is in contrast to the monotonic decay near the free surface for water films. This dielectric oscillation phenomenon is associated with the strong response to hard solid/water interface. It significantly suppresses the dielectric relaxation and slows down the rotational diffusion near the interface compared to bulk water. We propose that the rotational diffusion determines the active degree of growth sites on interfaces, and its slowdown due to the interfacial dielectric oscillation contributes to reducing the growth rate of ice. With this idea, we predict the crystal growth rate of ice based on the modified Wilson–Frenkel model involving rotational dynamics. The theoretical result agrees well with the simulation.

Crystallization in highly supercooled Cu₅₀Zr₅₀ films close to the glass transition is studied by using molecular dynamics simulations. Spontaneous nucleation is observed at the simulation timescale in contrast to the bulk counterpart. We find that nucleation occurs at free surfaces owing to the partial wetting of the nucleus by melt. The anomalous wetting phenomenon is closely related to strong density layering arising from the surface: the high density associated with surface layering increases surface energy of supercooled melts, resulting in that one facet of the crystalline embryo is preferentially formed on the film surface. The surface-based embryo is then developed into a stable nucleus by bridging two surfaces of thin films. The kinetics and thermodynamics analyses based on the mean first-passage time method show that the nucleation process still follows the description of the classical nucleation theory despite extremely high supercoolings. In nucleating, the slow interface dynamics becomes dominant and induces a low nucleation rate although the nucleation barrier is very low. The subsequent crystal growth is found to proceed in a quasi-two-dimensional manner with a ramified interface morphology, which is analogous to percolative crystals predicted in glass-forming liquids.