Deviation from equilibrium conditions in molecular dynamic simulations of homogeneous nucleation

We present a comparison between Monte Carlo (MC) results for homogeneous vapourliquid nucleation of Lennard-Jones clusters and previously published values from molecular dynamics (MD) simulations. Both the MC and MD methods sample real cluster configuration distributions. In the MD simulations the extent of the temperature fluctuation is usually controlled with an artificial thermostat rather than with more realistic carrier gas. In this study primarily velocity scaling thermostat is considered, but also Nos´e-Hoover, Berendsen and stochastic Langevin thermostat methods are covered. The nucleation rates based on a kinetic scheme and the canonical MC calculation serve as a point of reference since they by definition describe an equilibrated system. The studied temperature range is from T = 0.3 to 0.65 ϵ/k. The kinetic scheme reproduces well the isothermal nucleation rates obtained by Wedekind et al. [J. Chem. Phys. 127, 064501 (2007)] using MD simulations with carrier gas. The nucleation rates obtained by artificially thermostatted MD simulations are consistently lower than the reference nucleation rates based on MC calculations. The discrepancy increases up to several orders of magnitude when the density of the nucleating vapour decreases. At low temperatures the difference to the MC-based reference nucleation rates in some cases exceeds the maximal nonisothermal effect predicted by classical theory of Feder et al. [Adv. Phys. 15, 111 (1966)].Peer reviewe

Modeling on Fragmentation of Clusters inside a Mass Spectrometer

Atmospheric clusters are weakly bound and can fragment inside the measuring instruments, in particular, mass spectrometers. Since the clusters accelerate under electric fields, the fragmentation cannot be described in terms of rate constants under equilibrium conditions. Using basic statistical principles, we have developed a model for fragmentation of clusters moving under an external force. The model describes an energy transfer to the cluster internal modes caused by collisions with residual carrier gas molecules. As soon as enough energy is accumulated in the cluster internal modes, it can fragment. The model can be used for interpreting experimental measurements by atmospheric pressure interface mass spectrometers.Peer reviewe

How well can we predict cluster fragmentation inside a mass spectrometer?

Fragmentation of molecular clusters inside mass spectrometers is a significant source of uncertainty in a wide range of chemical applications. We have measured the fragmentation of sulfuric acid clusters driving atmospheric new-particle formation, and developed a novel model, based on first principles calculations, capable of quantitatively predicting the extent of fragmentation.Peer reviewe