ORCID Profile
0000-0002-8537-1921
Current Organisation
Tianjin University
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Publisher: Copernicus GmbH
Date: 09-2022
DOI: 10.5194/ACP-22-11155-2022
Abstract: Abstract. Ion–dipole collisions can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate the rate coefficients of ion–dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches under atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, s ling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles and charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physicochemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich (1982), which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and a central field model as a viable and elegant alternative to the brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.
Publisher: AIP Publishing
Date: 15-05-2023
DOI: 10.1063/5.0148013
Abstract: Molecular dynamics (MD) simulations of gas-phase chemical reactions are typically carried out on a small number of molecules near thermal equilibrium by means of various thermostatting algorithms. Correct equipartitioning of kinetic energy among translations, rotations, and vibrations of the simulated reactants is critical for many processes occurring in the gas phase. As thermalizing collisions are infrequent in gas-phase simulations, the thermostat has to efficiently reach equipartitioning in the system during equilibration and maintain it throughout the actual simulation. Furthermore, in non-equilibrium simulations where heat is released locally, the action of the thermostat should not lead to unphysical changes in the overall dynamics of the system. Here, we explore issues related to both obtaining and maintaining thermal equilibrium in MD simulations of an exemplary ion–molecule dimerization reaction. We first compare the efficiency of global (Nosé–Hoover and Canonical S ling through Velocity Rescaling) and local (Langevin) thermostats for equilibrating a system of flexible compounds and find that of these three only the Langevin thermostat achieves equipartition in a reasonable simulation time. We then study the effect of the unphysical removal of latent heat released during simulations involving multiple dimerization events. As the Langevin thermostat does not produce the correct dynamics in the free molecular regime, we only consider the commonly used Nosé–Hoover thermostat, which is shown to effectively cool down the reactants, leading to an overestimation of the dimerization rate. Our findings underscore the importance of thermostatting for the proper thermal initialization of gas-phase systems and the consequences of global thermostatting in non-equilibrium simulations.
Publisher: American Chemical Society (ACS)
Date: 10-05-2021
Publisher: Proceedings of the National Academy of Sciences
Date: 05-07-2022
Abstract: Nucleation of clusters from the gas phase is a widely encountered phenomenon, yet rather little is understood about the underlying out-of-equilibrium dynamics of this process. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial. To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics simulations of a supersaturated vapor in the presence of thermalizing carrier gas. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and classical nonisothermal theory. For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes and resolve the apparent contradictions regarding cluster temperatures. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15132
Abstract: & & The condensation of carbon dioxide (CO2) is a topic of general interest in view of global decarbonization targets, e.g. in low-temperature CO2 capture technologies promoting the phase transition of CO2 gas is the crucial step. Homogeneous nucleation of a mixture of CO2 and argon gas in a supersonic nozzle has been studied at temperatures from 78 to 92 K, and CO2 partial pressures between 70 and 800 Pa. The consistency between the current data and measurements at higher temperature suggests the critical clusters remain liquid-like even at these low temperatures.& & & & Here we present large-scale atomistic molecular dynamics (MD) simulations of homogenous CO2 nucleation from the vapour phase at temperatures from 75 to 105 K. The MD approach is an unbiased method to study the nucleation process, including the phase and structure of even the smallest clusters. We used argon carrier gas as a heat bath for the CO2 molecules to avoid unphysical removal of latent heat.& & & & Simulations confirm that despite strong undercooling, nucleation proceeds through liquid-like clusters. Also, by applying standard steady-state cluster growth kinetics, we are able to calculate the cluster formation free energies from the MD simulations. The results suggest a curvature correction to the classical liquid drop model used in the classical nucleation theory. The correction depends only on the bulk liquid properties, and hence the simulation-based correction can be applied to predict the nucleation rates of real CO2.& & & & The simulation-based theory is able to capture the magnitude and the temperature-dependency of the nucleation rate rather well, whereas both standard CNT and its self-consistent version (SCNT) underestimate the rate by several orders of magnitude. Here we have corrected the theoretical values with the non-isothermal factor, which is about 0.01-0.1 for the studied system.& &
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0CP02279A
Abstract: Small angle X-ray scattering and pressure measurements yield quantitative homogeneous nucleation rates for CO 2 near 80 K.
Publisher: Copernicus GmbH
Date: 11-06-2019
DOI: 10.5194/ACP-2019-400
Abstract: Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical non-interacting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We have studied the statistics of collisions of sulfuric acid molecules in vacuum by atomistic molecular dynamics simulations. We have found that the effective collision cross section of the H2SO4 molecule, as described by an OPLS-All Atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor 2.2, compared to kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, fitted to the attractive part of the atomistic intermolecular potential of mean force.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-14945
Abstract: Collisions between ions and dipolar molecules can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models or simple parametrizations have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners.& & & & & & & & To determine the accuracy and applicability of these theoretical approaches, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectory simulations, s ling a relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters.& & & & & & & & We find excellent agreement between the collision cross sections and rate coefficients obtained from the molecular dynamics trajectory simulations and the central field model. Therefore, we conclude that the effective interactions between the collision partners are highly isotropic, and the central field model is able to capture the relevant physicochemical properties of the system.& & & & & & & & Comparing the molecular dynamics trajectory simulations with the often-used parametrization by Su and Chesnavich (1982), we find that the latter can predict the collision rate coefficient quite well for systems with a molecular dipole, but the agreement worsens for systems with a dipolar cluster.& & & & & & & & Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.& & & & & & & & We are currently using the combination of potential of mean force calculation and central field model, as well as the atomistic trajectory simulations, to understand the relatively large increase in rate coefficient observed in chemical ionization mass spectrometers when sulfuric acid is charged with acetate, as compared to nitrate (Fomete, 2022).& Su, T. and Chesnavich, W. J.: J. Chem. Phys. 76, 5183& #8211 , 1982.Fomete. S. et al.: J. Phys. Chem. A 126, 44, 8240& #8211 , 2022.
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D0CP05653G
Abstract: Molecular dynamics simulations reveal the structural and energetic properties of carbon dioxide clusters nucleating in the gas phase at extreme undercooling.
Publisher: Copernicus GmbH
Date: 16-05-2022
DOI: 10.5194/EGUSPHERE-2022-93
Abstract: Abstract. Ion-dipole collisions can facilitate the formation of atmospheric aerosol particles, and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches at atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, s ling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physico-chemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich [J. Chem. Phys., 76, 5183–5185, 1982], which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force s ling of in idual collision trajectories over a large range of impact parameters and relative velocities.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-18959
Abstract: & & & span& Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical noninteracting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. & /span& We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS) all-atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid.& span& & As a consequence, the actual collision coefficient is enhanced by a factor of 2.2 at 300& #8201 K compared with kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard-sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, based on the attractive part of the atomistic intermolecular potential of mean force.& /span& & &
Publisher: Copernicus GmbH
Date: 30-10-2019
DOI: 10.5194/ACP-19-13355-2019
Abstract: Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical noninteracting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS). OPLS all-atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor of 2.2 at 300 K compared with kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard-sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, based on the attractive part of the atomistic intermolecular potential of mean force.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-12521
Abstract: Nucleation of clusters from the gas phase is a widely encountered phenomenon, e.g. regional air quality and global climate are both directly impacted by particle formation from atmospheric trace gases [1]. Still, the underlying out-of-equilibrium dynamics of this process are not well understood. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, as in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial.To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics (MD) simulations of a supersaturated Lennard-Jones (LJ) vapor in the presence of thermalizing carrier gas. In addition, a previous study of homogeneous nucleation of carbon dioxide in argon carrier gas [2] was revisited for temperature analysis of the growing CO2 & clusters. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and the classical nonisothermal theory by Feder et al. [3], which also predicts the existence of cool subcritical clusters, and has been quite controversial.For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are indeed elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes. This resolves the apparent contradiction between elevated cluster temperatures and minor nonisothermal corrections to the nucleation rate, both often reported in literature, and is in excellent agreement with the theory of Feder et al. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.& References[1] M. Kulmala et al., Direct observations of atmospheric aerosol nucleation. Science 339, 943& #8211 (2013).[2] R. Halonen et al., Homogeneous nucleation of carbon dioxide in supersonic nozzles II: Molecular dynamics simulations and properties of nucleating clusters. Phys. Chem. Chem. Phys. 23, 4517& #8211 (2021).[3] J. Feder, K. C. Russell, J. Lothe, G. M. Pound, Homogeneous nucleation and growth of droplets in vapours. Adv. Phys. 15, 111& #8211 (1966).
No related grants have been discovered for Roope Halonen.