Large scale molecular dynamics simulations of homogeneous nucleation

Abstract

We present results from large-scale molecular dynamics (MD) simulations of homogeneous vapor-to-liquid nucleation. The simulations contain between 1 × 109 and 8 × 109 Lennard-Jones (LJ) atoms, covering up to 1.2 s (56 × 106 time-steps). They cover a wide range of supersaturation ratios, S = 1.55-104, and temperatures from kT = 0.3 to 1.0 (where is the depth of the LJ potential, and k is the Boltzmann constant). We have resolved nucleation rates as low as 1017 cm-3 s-1 (in the argon system), and critical cluster sizes as large as 100 atoms. Recent argon nucleation experiments probe nucleation rates in an overlapping range, making the first direct comparison between laboratory experiments and molecular dynamics simulations possible: We find very good agreement within the uncertainties, which are mainly due to the extrapolations of argon and LJ saturation curves to very low temperatures. The self-consistent, modified classical nucleation model of Girshick and Chiu [J. Chem. Phys. 93, 1273 (1990)] underestimates the nucleation rates by up to 9 orders of magnitudes at low temperatures, and at kT = 1.0 it overestimates them by up to 105. The predictions from a semi-phenomenological model by Laaksonen et al. [Phys. Rev. E 49, 5517 (1994)] are much closer to our MD results, but still differ by factors of up to 104 in some cases. At low temperatures, the classical theory predicts critical clusters sizes, which match the simulation results (using the first nucleation theorem) quite well, while the semi-phenomenological model slightly underestimates them. At kT = 1.0 , the critical sizes from both models are clearly too small. In our simulations the growth rates per encounter, which are often taken to be unity in nucleation models, lie in a range from 0.05 to 0.24. We devise a new, empirical nucleation model based on free energy functions derived from subcritical cluster abundances, and find that it performs well in estimating nucleation rates. We present results from large-scale molecular dynamics (MD) simulations of homogeneous vapor-toliquid nucleation. The simulations contain between 1 × 10 9 and 8 × 10 9 Lennard-Jones (LJ) atoms, covering up to 1.2 μs (56 × 10 6 time-steps). They cover a wide range of supersaturation ratios, S 1.55-10 4 , and temperatures from kT = 0.3 to 1.0 (where is the depth of the LJ potential, and k is the Boltzmann constant). We have resolved nucleation rates as low as 10 17 cm −3 s −1 (in the argon system), and critical cluster sizes as large as 100 atoms. Recent argon nucleation experiments probe nucleation rates in an overlapping range, making the first direct comparison between laboratory experiments and molecular dynamics simulations possible: We find very good agreement within the uncertainties, which are mainly due to the extrapolations of argon and LJ saturation curves to very low temperatures. The self-consistent, modified classical nucleation model of Girshick and Chiu [J. Chem. Phys. 93, 1273 (1990)] underestimates the nucleation rates by up to 9 orders of magnitudes at l...

Analyzing multistep homogeneous nucleation in vapor-to-solid transitions using molecular dynamics simulations

“…Large-scale direct (NV E and NV T ) MD simulations with up to ∼10 9 molecules were recently reported [29,32,33]. Diemand et al [29] were able to resolve significantly lower nucleation rates for the vapor-to-liquid transition than past work, and they obtained the first results of MD nucleation that are directly comparable to those of laboratory experiments [29].…”

“…Diemand et al [29] were able to resolve significantly lower nucleation rates for the vapor-to-liquid transition than past work, and they obtained the first results of MD nucleation that are directly comparable to those of laboratory experiments [29]. In [29], however, crystallization of the liquid particles was not observed. With a longer period of integration, the large number of liquid supercooled nanoparticles forming naturally out of the vapor allow us to study particle crystallization in detail.…”

“…In the argon system, the units were /k = 119.8 K, σ = 3.405Å, m = 6.634 × 10 −23 g, and τ = 2.16 ps. The simulations box had periodic boundary conditions, and the time steps were set to t = 0.01τ = 0.01σ √ m/ , in the same way as the previous simulations for the vapor-to-liquid nucleation [29,30].…”

“…Clusters were defined using the Stillinger criterion with a search radius of 1.52 σ [29,30]. The crystal structure was identified using the common neighbor analysis (CNA) method [35,36] implemented in LAMMPS.…”

“…Although many studies have focused on vapor-to-liquid nucleation by MD simulations [23][24][25][26][27][28][29], few have considered vapor-to-solid nucleation [30,31]. A multistep nucleation for the vapor-to-solid transition was observed in [30], but no detailed analysis of the crystallization process was conducted.…”