Papers by Siegfried Fritzsche
ChemTexts, 2018
This lecture aims at advanced (master's, PhD) students in physics, chemistry, physical chemistry,... more This lecture aims at advanced (master's, PhD) students in physics, chemistry, physical chemistry, biochemistry, engineering (and possibly biology) who use, or plan to use, molecular dynamics (MD) computer simulations in the course of their research work. This lecture is, however, neither (or only in a very limited way) a course on the scientific background of this method (quantum mechanics, statistical mechanics, computational methods), nor is it a pragmatics tutorial ('how-to' guide) which button to click on some graphical interface or other. We rather aim at pointing out to the aspiring user of any kind of simulation software some of the important choices that must be made as well as some of the problems and pitfalls that he or she may encounter on the way to reliable and meaningful scientific results. This includes a few reminders what not to forget to avoid such mistakes and possibly where to look to correct them if they have, unavoidably, been made.
The Journal of Physical Chemistry C, 2017
Adsorption and diffusion of the gas mixture H 2 /CH 4 in the metal−organic framework (MOF) of str... more Adsorption and diffusion of the gas mixture H 2 /CH 4 in the metal−organic framework (MOF) of structure type zeolitic imidazolate framework-90 (ZIF-90) are revisited. While the adsorption can successfully be examined in Gibbs ensemble Monte Carlo (GEMC) simulations using the common approximation of a rigid lattice, the dynamics of methane in ZIF-90 is remarkably influenced by the lattice flexibility. Molecular dynamics (MD) simulations not only show a strong influence of the lattice flexibility on the diffusion of methane but even find a slight structural phase transition of the lattice. This structural change appeared at higher temperatures and was not caused by the content of guest molecules like in most former discoveries of gate opening. For prediction of mixed gas ZIF-90 membrane selectivity, the adsorption and diffusion results show that the high CH 4 adsorption selectivity is overcompensated by the high H 2 mobility. The comparison of our results for the H 2 /CH 4 membrane selectivity with experimental findings from mixed gas permeation through supported ZIF-90 membranes shows better agreement than other simulations that use a rigid lattice for MD. Also, the increase of the membrane selectivity by increased temperature could be found.
The Journal of Physical Chemistry C, 2007
Using classical molecular dynamics (MD) simulations, we have studied some structural and diffusiv... more Using classical molecular dynamics (MD) simulations, we have studied some structural and diffusive properties of water molecules adsorbed in chabazite. In particular, we have investigated the variation of the self-diffusion coefficient of the water molecules as a function of their concentration and the nature of the hydration shells of the extraframework Ca ++ ions with varying concentrations of water. Our study indicates that the welldefined and stable hydration shells of this ion play an important role in the diffusion process. The diffusion anisotropy is computed at T) 600 K. It is compared with theoretical results based on jump models and qualitatively compared with pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments of a single chabazite crystal at 293 K and with tracer diffusion studies.
Fig.2 The distribution of sorbates in the channels at various temperatures. The loading of sorbat... more Fig.2 The distribution of sorbates in the channels at various temperatures. The loading of sorbates is 4 molecules per unit cell for all temperatures.
Binding Energy of Guest Molecules on the Silanol Covered (010) Silicalite-1 Surface as Studied by Quanum Chemical Calculations
Diffusion in Channels and Channel Networks
Lecture Notes in Physics, 2004
A large variety of nanoporous materials host channel networks with pore diameters on the order of... more A large variety of nanoporous materials host channel networks with pore diameters on the order of typical molecular dimensions. Molecular diffusion and reaction in such systems exhibits a number of peculiarities, which are exemplified in more detail in this chapter. As a main feature of diffusion in channels with diameters small enough so that the individual diffusants cannot pass each other, their mean square displacement is found to increase with the square root of time rather than with the time itself, as to be required for normal diffusion. In channels of finite extension this peculiarity of single-file diffusion is soon masked by a second mechanism, stipulated by the fast particle exchange at the boundary, which leads to molecular displacements following the time dependence of normal diffusion. The interplay of these two processes is discussed in Sect. 2. In mutually intersecting arrays of channels, molecular diffusion in different directions may be correlated between each other. Section 3 in particular considers the question, up to which extent the memory of the diffusants may affect this correlation. If the different arrays have different affinities to the constituents of multicomponent systems, the rate of molecular reactions in such systems may be enhanced in comparison with systems, where all parts of the pore system are equally accessible by the involved components. The potentials and limits of reactivity enhancement by this effect of molecular transport control are discussed in Sect. 4. In addition to the examples of experimental reference, which are appropriately included in each of the individual sections, Sect. 5 is exclusively devoted to the question, how closely the textbook structure of nanoporous materials with channel arrays is approached by reality.
Grundlagen der Statistischen Physik
Molekulardynamik, 1995
Die zur Berechnung der gewunschten strukturellen, thermodynamischen und kinetischen Daten mittels... more Die zur Berechnung der gewunschten strukturellen, thermodynamischen und kinetischen Daten mittels Computersimulationen benotigten Ausgangsgrosen wie radiale Verteilungsfunktionen, Zustandssummen und Korrelationsfunktionen sowie die dabei benotigten Relationen werden durch die Statistische Physik — Statistische Thermodynamik im Gleichgewicht und Statistische Theorie irreversibler Prozesse im Nicht-Gleichgewicht — bereitgestellt. Die Grundlagen werden im folgenden Kapitel kurz zusammengestellt. Ausfuhrlichere Darstellungen entnimmt man der Literatur [14–18].
Der Verlag Vieweg ist ein Unternehmen der Bertelsmann Fachinformation GmbH. Das Werk einschließli... more Der Verlag Vieweg ist ein Unternehmen der Bertelsmann Fachinformation GmbH. Das Werk einschließlich aller seiner Teile ist urheberrechtlich geschützt. Jede Verwertung außerhalb der engen Grenzen des Urheberrechtsgesetzes ist ohne Zustimmung des Verlags unzulässig und strafbar. Das gilt insbesondere für Vervielfältigungen, Übersetzungen, Mikroverfilmungen und die Einspeicherung und Verarbeitung in elektronischen Systemen.
The Journal of Physical Chemistry B, 2003
A series of molecular dynamics simulations have been performed to examine changes to the structur... more A series of molecular dynamics simulations have been performed to examine changes to the structural and dynamical properties of water molecules in silicalite-1 as a function of temperature and loading. The ab initio fitted silicalite-1/water potential which is newly developed 18 and the BJH flexible water/water potential 15 have been employed. The water loading was varied from 1 to 8 water molecules per intersection, equivalent to 8-64 molecules per simulation cube. The simulations have been carried out at 298 and 393 K. The results show that the water structure inside the silicalite-1 cages changes dramatically as a function of loading. We found that the probability of water molecules residing in a straight channel is always higher than that of residing in the sinusoidal channels. Under high loading, the observed clusters form a structure similar to that of pure water. We call it a "low-density cluster" for the following reasons: (i) The cluster consists of five water molecules (four in the first hydration shell of the central one) which is consistent with that of pure water. (ii) However, molecules in the cluster are not coordinated together via hydrogen bonds. The radius of the first hydration shell of 3.35 Å is 0.5 Å longer than that of pure water. (iii) Molecules in the cluster are less flexible than those of pure water. In terms of dynamical properties, for low loadings, a preferential diffusion path is observed along the center of the channel tube. The water molecules were detected to diffuse closer to the surface when the concentration was higher than six molecules per intersection. The diffusion coefficient of water decreases when the concentration increases. The D values for all concentrations at 393 K are higher than those at 298 K. The temperature dependence almost disappears at a loading of eight water molecules per intersection. In addition, the anisotropic diffusion is less pronounced for water in silicalite-1 in comparison to that of nonpolar molecules.
The Journal of Physical Chemistry B, 1999
An approximation for the self-part of the van Hove function is derived. The self-part of the van ... more An approximation for the self-part of the van Hove function is derived. The self-part of the van Hove function is calculated by MD simulation for the diffusion of methane in ZK4 and silicalite and compared with the approximation. Fourier transform in space of the van Hove function yields the intermediate scattering function, the decay of which is used to determine the self-diffusion coefficient. Fourier transform in time yields the dynamic structure factor which can be compared with quasi-elastic neutron scattering results.
The Journal of Physical Chemistry B, 1998
The diffusion of a mixture of methane and xenon in the zeolite silicalite is studied by use of mo... more The diffusion of a mixture of methane and xenon in the zeolite silicalite is studied by use of molecular dynamics (MD) simulations and pulsed field gradient (PFG) nuclear magnetic resonance (NMR). For a fixed total number of guest molecules, the ratio of xenon to methane is varied in order to examine the special properties of diffusion in a mixture. High xenon concentrations were found to slow the methane diffusivity in the mixture, while the diffusion of xenon is nearly unaffected by high methane concentrations. The reason for the dominance of xenon is the larger local heat of adsorption of xenon and the larger mass of xenon compared to methane in combination with channel size and topology in silicalite. Simulated and experimental data are in very good agreement with each other. Diffusion anisotropy as resulting from the MD simulations is discussed in terms of the correlation rule for diffusion in the interconnected pore system of ZSM-5 (Kärger,
The Journal of Physical Chemistry A, 2009
The permeation of methane molecules through the silicalite-1 surfaces with and without silanol gr... more The permeation of methane molecules through the silicalite-1 surfaces with and without silanol groups has been studied by nonequilibrium molecular dynamics computer simulations. A newly fitted intermolecular potential between the methane molecules and the silanol is used. A control volume provides a nearly stationary gas phase close to the membrane. The nonequilibrium process of filling the (initially empty) membrane with methane molecules until saturation is considered, and the surface permeability has been evaluated. It turns out to be strongly influenced by the presence of silanol groups. Additionally it was found that for a large part of the loading process the particle stream into the zeolite membrane was nearly independent upon the deviation from equilibrium. This means that far from equilibrium the decay of this deviation does not follow an exponential law.
Microporous and Mesoporous Materials, 2009
Molecular dynamics simulations are employed to investigate pressure-driven water and ion transpor... more Molecular dynamics simulations are employed to investigate pressure-driven water and ion transport through a (9,9) carbon nanotube (CNT). We consider NaCl solutions modeled with both the TIP3P and TIP4P/2005 water models. Concentrations range from 0.25 to 2.8 mol l 1 and temperatures from 260 to 320 K are considered. We discuss the influences on flow rates of continuum hydrodynamic considerations and molecular structural effects. We show that the flow rate of water, sodium, and chloride ions through the CNT is strongly model dependent, consistent with earlier simulations of pure water conduction. To remove the effects of different water flow rates, and clearly expose the influence of other factors on ion flow, we calculate ion transport efficiencies. Ion transport efficiencies are much smaller for TIP4P/2005 solutions than for those using the TIP3P model. Particularly at lower temperatures, the ion transport efficiencies for the TIP4P/2005 model are small, despite the fact that the nanotube conducts water at a significant rate. We trace the origin of small ion transport efficiencies to the presence of ring-like water structures within the CNT. Such structures occur commonly for the TIP4P/2005 model, but less frequently for TIP3P. The water structure acts to reduce ion "solvation" within the CNT, posing an additional barrier to ion entry and transport. Our results demonstrate that increasing the water structure within the CNT by decreasing the temperature strongly inhibits ion conduction, while still permitting significant water transport.
Microporous and Mesoporous Materials, 2010
Microporous and Mesoporous Materials, 2011
Self-diffusion, libration and reorientation of water molecules in the zeolite chabazite are exami... more Self-diffusion, libration and reorientation of water molecules in the zeolite chabazite are examined by molecular dynamics (MD) computer simulations at different temperatures and loadings. Comparison with experiment provides satisfactory agreement of the self-diffusivities and excellent agreement of the anisotropy factors and explains the failure of their prediction by the concept of structure-correlated diffusion anisotropy. Long-range electrostatic interactions are found to affect self-diffusion at high loadings and temperatures. Spectral densities of librational motion are similar to those in aqueous salt solutions, while reorientation is much slower and much more anisotropic. The vector in direction of the molecular dipole moment reorients only very slowly, as a consequence of the attraction by the almost immobile Ca þþ-ions and by the walls of the zeolite. The other two vectors seem to reorient by jumps rather than by rotational diffusion.
The importance of lattice flexibility for the migration of ethane in ZIF-8: Molecular dynamics simulations
Microporous and Mesoporous Materials, 2013
ABSTRACT Recent adsorption, diffusion and permeation studies give experimental evidence that etha... more ABSTRACT Recent adsorption, diffusion and permeation studies give experimental evidence that ethane can easily adsorb and diffuse in the ZIF-8 framework. This finding is surprising since the pore window of ZIF-8 – derived from XRD data – is virtually too small for ethane migration. Therefore, the diffusion mechanism of ethane in ZIF-8 and the influence of the diffusing guest molecules on the flexible lattice are investigated in detail in this paper with focus on the diffusion limiting step, the passage of the ZIF-8 window between adjacent cavities. Contrary to zeolite research, well established interaction parameter sets are still not available for MOF simulations. Hence, a large part of this paper is devoted the comparison of several parameter sets for the molecular interactions. Also, the conservation not only of the unit box size in NPT simulations but also the conservation of the geometrical structure of the lattice (most important the “windows” between adjacent pores) has been examined. Employing a suitable parameter set, two interesting effects can be observed: (i) The window size of ZIF-8 is smaller at higher ethane loadings, while simultaneously (ii) at higher loadings the guest molecules within the cavity exert forces that press a given probe molecule toward the window. These two effects nearly compensate each other leading to a weak dependence of the self-diffusivity upon the concentration of guest molecules.However, no gate-opening mechanism by a torsional motion of the linker molecule methylimidazolate could be found in our MD simulations because of a too large activation energy for this motion.
Microporous and Mesoporous Materials, 2014
In the field of nanoporous materials, guest-induced structural changes in metal-organic framework... more In the field of nanoporous materials, guest-induced structural changes in metal-organic frameworks (MOFs) attracted great attention over the last years. One example concerns a gate-opening effect in MOF ZIF-8 which was found to occur upon sorption of N 2. To mirror these structural changes in molecular simulations, suitable force fields for the proper description of framework flexibility are required. We demonstrate that our previously published force field is able to reproduce these structural changes in classical MD simulations. In particular, with our parameter set the diameter of the windows connecting adjacent cavities is found to match the X-ray values almost perfectly. We focus on investigating the impact of the structural changes on the mobility of N 2 molecules in ZIF-8 framework and compare the results with those of another parameter set, which was published recently by Zhang and coworkers. The size of the windows increases notably, when the critical ''gate-opening'' loading is surpassed. Most surprisingly, this pronounced increase does not result in a speed-up of the N 2 self-diffusivity. It appears, that a complex interplay of host-host and host-guest interactions increases the mutual hindrance of the N 2 molecules and counter balance the acceleration of the mobility due to the larger window size.
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Papers by Siegfried Fritzsche