Search for: All records

Molecular dynamics simulations are typically constrained to have a fixed number of particles, which limits our capability to simulate chemical and physical processes where the composition of the system changes during the simulation time. Typical examples are the calculation of nucleation and crystal growth rates in heterogeneous solutions where the driving force depends on the composition of the fluid. Constant chemical potential molecular dynamics simulations would instead be required to compute time-independent growth and nucleation rates. While this can, in principle, be achieved through the addition and deletion of particles using the grand canonical partition function, this is very inefficient in the condensed phase due to the low acceptance probability of these events. Adaptive resolution schemes, which use a reservoir of non-interacting particles that can be transformed into solute particles, circumvent this problem, but at the cost of relatively complicated code implementations. In this work, a simpler approach is proposed that uses harmonic volumetric restraints to control the solute osmotic pressure, which can be considered a proxy for the system’s chemical potential. The osmotic pressure regulator is demonstrated to reproduce the expected properties of ideal gases and ideal solutions. Using the mW water model, the osmotic pressure regulator is shown to provide a constant growth rate for ice in the presence of an electrolyte solution, unlike what standard molecular dynamics simulations would produce. 

Abstract Experimental challenges in determining the phase diagram of carbon at temperatures and pressures near the graphite-diamond-liquid triple point are often related to the persistence of metastable crystalline or glassy phases, superheated crystals, or supercooled liquids. A deeper understanding of the crystallisation kinetics of diamond and graphite is crucial for effectively interpreting the outcomes of these experiments. Here, we reveal the microscopic mechanisms of diamond and graphite nucleation from liquid carbon through molecular simulations with first-principles machine learning potentials. Our simulations accurately reproduce the experimental phase diagram of carbon near the triple point and show that liquid carbon crystallises spontaneously upon cooling. Metastable graphite crystallises in the domain of diamond thermodynamic stability at pressures above the triple point. Furthermore, whereas diamond crystallises through a classical nucleation pathway, graphite follows a two-step process in which low-density fluctuations forego ordering. Calculations of the nucleation rates of the two competing phases confirm this result and reveal a manifestation of Ostwald’s step rule, where the strong metastability of graphite hinders the transformation to the stable diamond phase. Our results provide a key to interpreting melting and recrystallisation experiments and shed light on nucleation kinetics in polymorphic materials with deep metastable states. 

The nitrate anion (NO3−) is abundant in environmental aqueous phases, including aerosols, surface waters, and snow, where its photolysis releases nitrogen oxides back into the atmosphere. Nitrate photolysis occurs via two channels: (1) the formation of NO2 and O− and (2) the formation of NO2− and O(3P). The occurrence of two reaction channels with very low quantum yield (∼1%) highlights the critical role of the solvation environment and spin-forbidden electronic transitions, which remain unexplained at the molecular level. We investigate the two photolysis channels in water using quantum chemical calculations and first-principles molecular dynamics simulations with hybrid density functional theory and enhanced sampling. We find that spin-forbidden absorption to the triplet state (T1) is possible but occurs at a rate ∼15 times weaker than the spin-allowed transition to the singlet state (S1). A metastable solvation cage complex requires additional thermal energy to dissociate the N–O bond, allowing for recombination or non-radiative deactivation. Our results explain the temperature dependence of photolysis, linked to hydrogen bond rearrangement in the solvation shell. This work provides new molecular insights into nitrate photolysis and its low quantum yield under environmental conditions. 

Abstract Liquid water can be supercooled up to about 50~K below the melting point before undergoing homogeneous ice nucleation. Based on experimental thermodynamic observations and computer simulations it was hypothesized that below this temperature and at pressures of several kbar water undergoes a liquid-liquid phase transition (LLPT) and the transition line ends at a second critical point. However, challenges in experiments and simulations at such deep cooling leave doubts about the nature of the LLPT and the existence of the critical point.Here we use molecular dynamics simulations with a highly accurate and computationally efficient polarizable water model to establish the character of the LLPT and identify the location of the second critical point. Our microsecond-long simulations provide the first direct evidence of a well-defined moving interface between low-density and high-density water at conditions near the phase boundary. This is the ultimate proof of a first-order transition between two liquid phases with distinct free energy basins separated by a barrier, resolving a long-standing debate. These results provide new perspectives on supercooled water under pressure simulated with an accurate and realistic model suitable for studies of water in confined geological and biological environments. 

We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na + /Cl − ) ions by molecular dynamics simulations. We find that the presence of a high surface density of Na + /Cl − pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to a low-temperature roughening transition of the air–ice interface. The local molecular structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na + /Cl − pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions.