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Despite the importance of ice nucleation, this process has been barely explored at negative pressures. Here, we study homogeneous ice nucleation in stretched water by means of Molecular Dynamics Seeding simulations using the TIP4P/Ice model. We observe that the critical nucleus size, interfacial free energy, free energy barrier, and nucleation rate barely change between isobars from -2600 to 500 bar when they are represented as a function of supercooling. This allows us to identify universal empirical expressions for homogeneous ice nucleation in the pressure range from -2600 to 500 bar. We show that this universal behavior arises from the pressure dependence of the interfacial free energy which we compute by means of the mold integration technique finding a shallow minimum around -2000 bar. Likewise, we show that the change in the interfacial free energy with pressure is proportional to the excess entropy and the slope of the melting line, exhibiting the latter a reentrant behavior also at the same negative pressure. Finally, we estimate the excess energy and the excess entropy of the ice Ih-water interface.

Using the crystallization transition in a Lennard-Jones fluid as example, we show that mean first-passage time based methods may underestimate the reaction rates. We trace the reason of this deficiency back to the non-Markovian character of the dynamics caused by the projection to a poorly chosen reaction coordinate. The non-Markovianity of the dynamics becomes apparent in the behavior of the recurrence times., Comment: 7 pages, 5 figures

Small pre-structured seeds introduced into an undercooled fluid are known to increase the crystal nucleation rate by some orders of magnitude, if the structure of the seeds is commensurate with the bulk crystalline phase. The presence of such seeds also alters the crystallisation mechanism by favouring particular structures at the early stages of the nucleation process. Here, we study with computer simulations the effect of small face-centred cubic and body-centred cubic seeds on the crystallisation of a Lennard-Jones liquid in the strongly undercooled regime. We find that seeds with body-centred cubic structure lead to a larger enhancement of the crystallisation rate than face-centred cubic seeds. An analysis of recurrence times reveals that the size of the largest crystalline cluster used as reaction coordinate is affected by pronounced memory effects, which depend on the particular seed structure and point to the importance of structural information in the definition of a good reaction coordinate for crystallisation., Comment: 7 pages, 4 figures

Understanding the thermally activated escape from a metastable state is at the heart of important phenomena such as the folding dynamics of proteins, the kinetics of chemical reactions or the stability of mechanical systems. In 1940 Kramers calculated escape rates both in the high damping and the low damping regime and suggested that the rate must have a maximum for intermediate damping. This phenomenon, today known as the Kramers turnover, has triggered important theoretical and numerical studies. However, to date there is no direct and quantitative experimental verification of this turnover. Using a nanoparticle trapped in a bi-stable optical potential we experimentally measure the nanoparticle's transition rates for variable damping and directly resolve the Kramers turnover. Our measurements are in agreement with an analytical model that is free of adjustable parameters.

We propose a new algorithm for non-equilibrium molecular dynamics simulations of thermal gradients. The algorithm is an extension of the heat exchange algorithm developed by Hafskjold and co-workers [Mol. Phys. 80, 1389 (1993); Mol. Phys. 81, 251 (1994)], in which a certain amount of heat is added to one region and removed from another by rescaling velocities appropriately. Since the amount of added and removed heat is the same and the dynamics between velocity rescaling steps is Hamiltonian, the heat exchange algorithm is expected to conserve the energy. However, it has been reported previously that the original version of the heat exchange algorithm exhibits a pronounced drift in the total energy, the exact cause of which remained hitherto unclear. Here, we show that the energy drift is due to the truncation error arising from the operator splitting and suggest an additional coordinate integration step as a remedy. The new algorithm retains all the advantages of the original one whilst exhibiting excellent energy conservation as illustrated for a Lennard-Jones liquid and SPC/E water., Comment: 8 pages, 6 figures

The kinetics of collective rearrangements in solution, such as protein folding and nanocrystal phase transitions, often involve free energy barriers that are both long and rough. Applying methods of transition path sampling to harvest simulated trajectories that exemplify such processes is typically made difficult by a very low acceptance rate for newly generated trajectories. We address this problem by introducing a new generation algorithm based on the linear short-time behavior of small disturbances in phase space. Using this ``precision shooting'' technique, arbitrarily small disturbances can be propagated in time, and any desired acceptance ratio of shooting moves can be obtained. We demonstrate the method for a simple but computationally problematic isomerization process in a dense liquid of soft spheres. We also discuss its applicability to barrier crossing events involving metastable intermediate states., Comment: 9 pages, 12 figures, submitted to J. Chem. Phys