@phdthesis{Melani2019, author = {Melani, Giacomo}, title = {From structural fluctuations to vibrational spectroscopy of adsorbates on surfaces}, doi = {10.25932/publishup-44182}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-441826}, school = {Universit{\"a}t Potsdam}, pages = {119}, year = {2019}, abstract = {Aluminum oxide is an Earth-abundant geological material, and its interaction with water is of crucial importance for geochemical and environmental processes. Some aluminum oxide surfaces are also known to be useful in heterogeneous catalysis, while the surface chemistry of aqueous oxide interfaces determines the corrosion, growth and dissolution of such materials. In this doctoral work, we looked mainly at the (0001) surface of α-Al 2 O 3 and its reactivity towards water. In particular, a great focus of this work is dedicated to simulate and address the vibrational spectra of water adsorbed on the α-alumina(0001) surface in various conditions and at different coverages. In fact, the main source of comparison and inspiration for this work comes from the collaboration with the "Interfacial Molecular Spectroscopy" group led by Dr. R. Kramer Campen at the Fritz-Haber Institute of the MPG in Berlin. The expertise of our project partners in surface-sensitive Vibrational Sum Frequency (VSF) generation spectroscopy was crucial to develop and adapt specific simulation schemes used in this work. Methodologically, the main approach employed in this thesis is Ab Initio Molecular Dynamics (AIMD) based on periodic Density Functional Theory (DFT) using the PBE functional with D2 dispersion correction. The analysis of vibrational frequencies from both a static and a dynamic, finite-temperature perspective offers the ability to investigate the water / aluminum oxide interface in close connection to experiment. The first project presented in this work considers the characterization of dissociatively adsorbed deuterated water on the Al-terminated (0001) surface. This particular structure is known from both experiment and theory to be the thermodynamically most stable surface termination of α-alumina in Ultra-High Vacuum (UHV) conditions. Based on experiments performed by our colleagues at FHI, different adsorption sites and products have been proposed and identified for D 2 O. While previous theoretical investigations only looked at vibrational frequencies of dissociated OD groups by staticNormal Modes Analysis (NMA), we rather employed a more sophisticated approach to directly assess vibrational spectra (like IR and VSF) at finite temperature from AIMD. In this work, we have employed a recent implementation which makes use of velocity-velocity autocorrelation functions to simulate such spectral responses of O-H(D) bonds. This approach allows for an efficient and qualitatively accurate estimation of Vibrational Densities of States (VDOS) as well as IR and VSF spectra, which are then tested against experimental spectra from our collaborators. In order to extend previous work on unimolecularly dissociated water on α-Al 2 O 3 , we then considered a different system, namely, a fully hydroxylated (0001) surface, which results from the reconstruction of the UHV-stable Al-terminated surface at high water contents. This model is then further extended by considering a hydroxylated surface with additional water molecules, forming a two-dimensional layer which serves as a potential template to simulate an aqueous interface in environmental conditions. Again, employing finite-temperature AIMD trajectories at the PBE+D2 level, we investigated the behaviour of both hydroxylated surface (HS) and the water-covered structure derived from it (known as HS+2ML). A full range of spectra, from VDOS to IR and VSF, is then calculated using the same methodology, as described above. This is the main focus of the second project, reported in Chapter 5. In this case, comparison between theoretical spectra and experimental data is definitely good. In particular, we underline the nature of high-frequency resonances observed above 3700 cm -1 in VSF experiments to be associated with surface OH-groups, known as "aluminols" which are a key fingerprint of the fully hydroxylated surface. In the third and last project, which is presented in Chapter 6, the extension of VSF spectroscopy experiments to the time-resolved regime offered us the opportunity to investigate vibrational energy relaxation at the α-alumina / water interface. Specifically, using again DFT-based AIMD simulations, we simulated vibrational lifetimes for surface aluminols as experimentally detected via pump-probe VSF. We considered the water-covered HS model as a potential candidate to address this problem. The vibrational (IR) excitation and subsequent relaxation is performed by means of a non-equilibrium molecular dynamics scheme. In such a scheme, we specifically looked at the O-H stretching mode of surface aluminols. Afterwards, the analysis of non-equilibrium trajectories allows for an estimation of relaxation times in the order of 2-4 ps which are in overall agreement with measured ones. The aim of this work has been to provide, within a consistent theoretical framework, a better understanding of vibrational spectroscopy and dynamics for water on the α-alumina(0001) surface,ranging from very low water coverage (similar to the UHV case) up to medium-high coverages, resembling the hydroxylated oxide in environmental moist conditions.}, language = {en} } @article{LaunDuffusWahlefeldetal.2022, author = {Laun, Konstantin and Duffus, Benjamin R. and Wahlefeld, Stefan and Katz, Sagie and Belger, Dennis Heinz and Hildebrandt, Peter and Mroginski, Maria Andrea and Leimk{\"u}hler, Silke and Zebger, Ingo}, title = {Infrared spectroscopy flucidates the inhibitor binding sites in a metal-dependent formate dehydrogenase}, series = {Chemistry - a European journal}, journal = {Chemistry - a European journal}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.202201091}, pages = {8}, year = {2022}, abstract = {Biological carbon dioxide (CO2) reduction is an important step by which organisms form valuable energy-richer molecules required for further metabolic processes. The Mo-dependent formate dehydrogenase (FDH) from Rhodobacter capsulatus catalyzes reversible formate oxidation to CO2 at a bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor. To elucidate potential substrate binding sites relevant for the mechanism, we studied herein the interaction with the inhibitory molecules azide and cyanate, which are isoelectronic to CO2 and charged as formate. We employed infrared (IR) spectroscopy in combination with density functional theory (DFT) and inhibition kinetics. One distinct inhibitory molecule was found to bind to either a non-competitive or a competitive binding site in the secondary coordination sphere of the active site. Site-directed mutagenesis of key amino acid residues in the vicinity of the bis-MGD cofactor revealed changes in both non-competitive and competitive binding, whereby the inhibitor is in case of the latter interaction presumably bound between the cofactor and the adjacent Arg587.}, language = {en} } @article{UtechtKlamroth2018, author = {Utecht, Manuel Martin and Klamroth, Tillmann}, title = {Local resonances in STM manipulation of chlorobenzene on Si(111)-7x7}, series = {Molecular physics}, volume = {116}, journal = {Molecular physics}, number = {13}, publisher = {Routledge, Taylor \& Francis Group}, address = {Abingdon}, issn = {0026-8976}, doi = {10.1080/00268976.2018.1442939}, pages = {1687 -- 1696}, year = {2018}, abstract = {Hot localised charge carriers on the Si(111)-7x7 surface are modelled by small charged clusters. Such resonances induce non-local desorption, i.e. more than 10 nm away from the injection site, of chlorobenzene in scanning tunnelling microscope experiments. We used such a cluster model to characterise resonance localisation and vibrational activation for positive and negative resonances recently. In this work, we investigate to which extent the model depends on details of the used cluster or quantum chemistry methods and try to identify the smallest possible cluster suitable for a description of the neutral surface and the ion resonances. Furthermore, a detailed analysis for different chemisorption orientations is performed. While some properties, as estimates of the resonance energy or absolute values for atomic changes, show such a dependency, the main findings are very robust with respect to changes in the model and/or the chemisorption geometry. [GRAPHICS] .}, language = {en} } @misc{UtechtKlamroth2018, author = {Utecht, Manuel Martin and Klamroth, Tillmann}, title = {Local resonances in STM manipulation of chlorobenzene on Si(111)-7×7}, series = {Molecular Physics}, journal = {Molecular Physics}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-412970}, pages = {11}, year = {2018}, abstract = {Hot localised charge carriers on the Si(111)-7×7 surface are modelled by small charged clusters. Such resonances induce non-local desorption, i.e. more than 10 nm away from the injection site, of chlorobenzene in scanning tunnelling microscope experiments. We used such a cluster model to characterise resonance localisation and vibrational activation for positive and negative resonances recently. In this work, we investigate to which extent the model depends on details of the used cluster or quantum chemistry methods and try to identify the smallest possible cluster suitable for a description of the neutral surface and the ion resonances. Furthermore, a detailed analysis for different chemisorption orientations is performed. While some properties, as estimates of the resonance energy or absolute values for atomic changes, show such a dependency, the main findings are very robust with respect to changes in the model and/or the chemisorption geometry.}, language = {en} } @article{KrivenkovGoliasMarchenkoetal.2017, author = {Krivenkov, Maxim and Golias, Evangelos and Marchenko, Dmitry and Sanchez-Barriga, Jaime and Bihlmayer, Gustav and Rader, Oliver and Varykhalov, Andrei}, title = {Nanostructural origin of giant Rashba effect in intercalated graphene}, series = {2D Materials}, volume = {4}, journal = {2D Materials}, number = {3}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2053-1583}, doi = {10.1088/2053-1583/aa7ad8}, pages = {11}, year = {2017}, abstract = {To enhance the spin-orbit interaction in graphene by a proximity effect without compromising the quasi-free-standing dispersion of the Dirac cones means balancing the opposing demands for strong and weak graphene-substrate interaction. So far, only the intercalation of Au under graphene/Ni(111) has proven successful, which was unexpected since graphene prefers a large separation (similar to 3.3 angstrom) from a Au monolayer in equilibrium. Here, we investigate this system and find the solution in a nanoscale effect. We reveal that the Au largely intercalates as nanoclusters. Our density functional theory calculations show that the graphene is periodically stapled to the Ni substrate, and this attraction presses graphene and Au nanoclusters together. This, in turn, causes a Rashba effect of the giant magnitude observed in experiment. Our findings show that nanopatterning of the substrate can be efficiently used for engineering of spin-orbit effects in graphene.}, language = {en} } @phdthesis{Rietze2020, author = {Rietze, Clemens}, title = {Optimierung und Analyse von molekularen Schaltern in komplexen Umgebungen: thermische Stabilit{\"a}t, Auslesbarkeit und Schaltbarkeit}, doi = {10.25932/publishup-45959}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-459594}, school = {Universit{\"a}t Potsdam}, pages = {131}, year = {2020}, abstract = {Seit Jahrzehnten stellen die molekularen Schalter ein wachsendes Forschungsgebiet dar. Im Rahmen dieser Dissertation stand die Verbesserung der thermischen Stabilit{\"a}t, der Auslesbarkeit und Schaltbarkeit dieser molekularen Schalter in komplexen Umgebungen mithilfe computergest{\"u}tzter Chemie im Vordergrund. Im ersten Projekt wurde die Kinetik der thermischen E → Z-Isomerisierung und die damit verbundene thermische Stabilit{\"a}t eines Azobenzol-Derivats untersucht. Daf{\"u}r wurde Dichtefunktionaltheorie (DFT) in Verbindung mit der Eyring-Theorie des {\"U}bergangszustandes (TST) angewendet. Das Azobenzol-Derivat diente als vereinfachtes Modell f{\"u}r das Schalten in einer komplexen Umgebung (hier in metallorganischen Ger{\"u}sten). Es wurden thermodynamische und kinetische Gr{\"o}ßen unter verschiedenen Einfl{\"u}ssen berechnet, wobei gute {\"U}bereinstimmungen mit dem Experiment gefunden wurden. Die hier verwendete Methode stellte einen geeigneten Ansatz dar, um diese Gr{\"o}ßen mit angemessener Genauigkeit vorherzusagen. Im zweiten Projekt wurde die Auslesbarkeit der Schaltzust{\"a}nde in Form des nichtlinearen optischen (NLO) Kontrastes f{\"u}r die Molek{\"u}lklasse der Fulgimide untersucht. Die daf{\"u}r ben{\"o}tigten dynamischen Hyperpolarisierbarkeiten unter Ber{\"u}cksichtigung der Elektronenkorrelation wurden mittels einer etablierten Skalierungsmethode berechnet. Es wurden verschiedene Fulgimide analysiert, wobei viele experimentelle Befunde best{\"a}tigt werden konnten. Dar{\"u}ber hinaus legte die theoretische Vorhersage f{\"u}r ein weiteres System nahe, dass insbesondere die Erweiterung des π-Elektronensystems ein vielversprechender Ansatz zur Verbesserung von NLO-Kontrasten darstellt. Die Fulgimide verf{\"u}gen somit {\"u}ber n{\"u}tzliche Eigenschaften, sodass diese in Zukunft als Bauelemente in photonischen und optoelektronischen Bereichen Anwendungen finden k{\"o}nnten. Im dritten Projekt wurde die E → Z-Isomerisierung auf ein quantenmechanisch (QM) behandeltes Dimer mit molekularmechanischer (MM) Umgebung und zwei Fluorazobenzol-Monomeren durch Molek{\"u}ldynamik simuliert. Dadurch wurde die Schaltbarkeit in komplexer Umgebung (hier selbstorgansierte Einzelschichten = SAMs) bzw. von Azobenzolderivaten analysiert. Mit dem QM/MM Modell wurden sowohl Van-der-Waals-Interaktionen mit der Umgebung als auch elektronische Kopplung (nur zwischen QM-Molek{\"u}len) ber{\"u}cksichtigt. Dabei wurden systematische Untersuchungen zur Packungsdichte durchgef{\"u}hrt. Es zeigte sich, dass bereits bei einem Molek{\"u}labstand von 4.5 {\AA} die Quantenausbeute (prozentuale Anzahl erfolgreicher Schaltprozesse) des Monomers erreicht wird. Die gr{\"o}ßten Quantenausbeuten wurden f{\"u}r die beiden untersuchten Fluorazobenzole erzielt. Es wurden die Effekte des Molek{\"u}labstandes und der Einfluss von Fluorsubstituenten auf die Dynamik eingehend untersucht, sodass der Weg f{\"u}r darauf aufbauende Studien geebnet ist.}, language = {de} } @article{LindicSinhaMattssonetal.2022, author = {Lindic, Tilen and Sinha, Shreya and Mattsson, Stefan and Paulus, Beate}, title = {Prediction of a model crystal structure for Ni2F5 by first-principles calculations}, series = {Zeitschrift f{\"u}r Naturforschung : B, Chemical sciences}, volume = {77}, journal = {Zeitschrift f{\"u}r Naturforschung : B, Chemical sciences}, number = {6}, publisher = {De Gruyter}, address = {Berlin}, issn = {0932-0776}, doi = {10.1515/znb-2022-0072}, pages = {469 -- 473}, year = {2022}, abstract = {Electrochemical fluorination in anhydrous HF, also known as the Simons process, is a widely used industrial method for fluorination of organic compounds. Its mechanism, being not so well understood, has long been debated and is believed to involve higher valent nickel fluorides formed on the nickel-plated anode during the process. One of these is speculated to be Ni2F5, which was previously reported in the literature and assigned via infrared spectroscopy, but its crystal structure is not yet known. We have identified known crystal structures of compounds with similar stoichiometries as Ni2F5 and utilized them as a starting point for our periodic DFT investigations, applying the PBE+U method. Ni2F5 as the most stable polymorph was found to be of the same crystal structure as another mixed valent fluoride, Cr2F5. The calculated lattice parameters are a = 7.24 angstrom, b = 7.40 angstrom, c = 7.08 angstrom and beta = 118.9 degrees with an antiferromagnetic ordering of the nickel magnetic moments.}, language = {en} } @phdthesis{Ehlert2016, author = {Ehlert, Christopher}, title = {Simulationen von R{\"o}ntgenabsorptionsprozessen zur Charakterisierung von Systemen in kondensierter Phase}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-104844}, school = {Universit{\"a}t Potsdam}, pages = {142}, year = {2016}, abstract = {Die vorgelegte Dissertation pr{\"a}sentiert wissenschaftliche Ergebnisse, die in der Zeit vom Dezember 2012 bis August 2016, erarbeitet wurden. Der zentrale Inhalt der Arbeit ist die Simulation von R{\"o}ntgenabsorptionsprozessen von verschiedenen Systemen in kondensierter Phase. Genauer gesagt, werden Nahkantenabsorptions- (NEXAFS) sowie R{\"o}ntgenphotoelektronenspektren (XPS) berechnet. In beiden F{\"a}llen wird ein R{\"o}ntgenphoton von einem molekularen System absorbiert. Aufgrund der hohen Photonenenergie wird ein stark gebundenes kernnahes Elektron angeregt. Bei der XPS gelangt dieses mit einer zu messenden kinetischen Energie in Kontinuumszust{\"a}nde. In Abh{\"a}ngigkeit der eingestrahlten Photonenenergie und der kinetischen Energie des austreten Elektrons, kann die Bindungsenergie berechnet werden, welche die zentrale Gr{\"o}ße der XPS ist. Im Falle der NEXAFS-Spektroskopie wird das kernnahe Elektron in unbesetzte gebundene Zust{\"a}nde angeregt. Die zentrale Gr{\"o}ße ist die Absorption als Funktion der eingestrahlten Photonenenergie. Das erste Kapitel meiner Arbeit er{\"o}rtert detailliert die experimentellen Methoden sowie die daraus gewonnenen charakteristischen Gr{\"o}ßen. Die experimentellen Spektren zeigen oft viele Resonanzen, deren Interpretation aufgrund fehlender Referenzmaterialien schwierig ist. In solchen F{\"a}llen bietet es sich an, die Spektren mittels quantenchemischer Methoden zu simulieren. Der daf{\"u}r erforderliche mathematisch-physikalische Methodenkatalog wird im zweiten Kapitel der Arbeit er{\"o}rtert. Das erste von mir untersuchte System ist Graphen. In experimentellen Arbeiten wurde die Oberfl{\"a}che mittels Bromplasma modifiziert. Die im Anschluss gemessenen NEXAFS-Spektren unterscheiden sich maßgeblich von den Spektren der unbehandelten Oberfl{\"a}che. Mithilfe periodischer DFT-Rechnungen wurden verschiedene Gitterdefekte sowie bromierte Systeme untersucht und die NEXAFS-Spektren simuliert. Mittels der Simulationen k{\"o}nnen die Beitr{\"a}ge verschiedener Anregungszentren analysiert werden. Die Berechnungen erlauben den Schluss, dass Gitterdefekte maßgeblich f{\"u}r die entstandenen Ver{\"a}nderungen verantwortlich sind. Polyvinylalkohol (PVA) wurde als zweites System behandelt. Hierbei sollte untersucht werden, wie groß der Einfluss der Molekularbewegung auf die Verbreiterung der Peaks im XP-Spektrum ist. Des Weiteren wurde untersucht, wie groß der Einfluss von intermolekularen Wechselwirkungen auf die Peakpositionen und Peakverbreiterung ist. F{\"u}r die Berechnung dieses Systems wurde eine Kombination aus molekulardynamischen und quantenchemischen Methoden verwendet. Als Strukturen dienten Oligomermodelle, die unter dem Einfluss eines (ab initio) Potentials propagiert wurden. Entlang der erstellten Trajektorie wurden Schnappsch{\"u}sse der Geometrien extrahiert und f{\"u}r die Berechnung der XP-Spektren verwendet. Die Spektren werden bereits mithilfe klassischer Molekulardynamik sehr gut reproduziert. Die erhaltenen Peakbreiten sind verglichen mit dem Experiment allerdings zu klein. Die Hauptursache der Peakverbreiterung ist die Molekularbewegung. Intermolekulare Wechselwirkungen verschieben die Peakpositionen um 0.6 eV zu kleineren Anregungsenergien. Im dritten Teil der Arbeit stehen die NEXAFS-Spektren von ionischen Fl{\"u}ssigkeiten (ILs) im Fokus. Die experimentell gefundenen Spektren zeigen eine komplexe Struktur mit vielen Resonanzen. In der Arbeit wurden zwei ILs untersucht. Als Geometrien verwenden wir Clustermodelle, die aus experimentellen Kristallstrukturen extrahiert wurden. Die berechneten Spektren erlauben es, die Resonanzen den Anregungszentren zuzuordnen. Außerdem kann eine erstmals gemessene Doppelresonanz simuliert und erkl{\"a}rt werden. Insgesamt kann die Interpretation der Spektren mithilfe der Simulation signifikant erweitert werden. In allen Systemen wurde zur Berechnung des NEXAFS-Spektrums eine auf Dichtefunktionaltheorie basierende Methode verwendet (die sogenannte Transition-Potential Methode). G{\"a}ngige wellenfunktionsbasierte Methoden, wie die Konfigurationswechselwirkung mit Einfachanregungen (CIS), zeigen eine starke Blauverschiebung, wenn als Referenz eine Hartree-Fock Slaterdeterminante verwendet wird. Wir zeigen, dass die Verwendung von kernnah-angeregten Determinanten sowohl das resultierende Spektrum als auch die Anregungsenergien deutlich verbessert. Des Weiteren werden auch Referenzen aus Dichtefunktionalrechnungen getestet. Zus{\"a}tzlich werden auch Referenzen mit gebrochenen Besetzungszahlen f{\"u}r kernnahe Elektronen verwendet. In der Arbeit werden die Resultate der verschiedenen Referenzen miteinander verglichen. Es zeigt sich, dass Referenzen mit gebrochenen Besetzungszahlen das Spektrum nicht weiter verbessern. Der Einfluss der verwendeten Elektronenstrukturmethode ist eher gering.}, language = {de} } @article{TaranNunezValdezEfthimiopoulosetal.2019, author = {Taran, Michail N. and Nunez Valdez, Maribel and Efthimiopoulos, Ilias and M{\"u}ller, J. and Reichmann, Hans-Josef and Wilke, Max and Koch-M{\"u}ller, Monika}, title = {Spectroscopic and ab initio studies of the pressure-induced Fe2+ high-spin-to-low-spin electronic transition in natural triphylite-lithiophilite}, series = {Physics and Chemistry of Minerals}, volume = {46}, journal = {Physics and Chemistry of Minerals}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0342-1791}, doi = {10.1007/s00269-018-1001-y}, pages = {245 -- 258}, year = {2019}, abstract = {Using optical absorption and Raman spectroscopic measurements, in conjunction with the first-principles calculations, a pressure-induced high-spin (HS)-to-low-spin (LS) state electronic transition of Fe2+ (M2-octahedral site) was resolved around 76-80GPa in a natural triphylite-lithiophilite sample with chemical composition (LiFe0.7082+Mn0.292PO4)-Li-M1-Fe-M2 (theoretical composition (LiFe0.52+Mn0.5PO4)-Li-M1-Fe-M2). The optical absorption spectra at ambient conditions consist of a broad doublet band with two constituents (1) (similar to 9330cm(-1)) and (2) (similar to 7110cm(-1)), resulting from the electronic spin-allowed transition (T2gEg)-T-5-E-5 of octahedral (HSFe2+)-Fe-M2. Both (1) and (2) bands shift non-linearly with pressure to higher energies up to similar to 55GPa. In the optical absorption spectrum measured at similar to 81GPa, the aforementioned HS-related bands disappear, whereas a new broadband with an intensity maximum close to 16,360cm(-1) appears, superimposed on the tail of the high-energy ligand-to-metal O2-Fe2+ charge-transfer absorption edge. We assign this new band to the electronic spin-allowed dd-transition (1)A(1g)(1)T(1g) of LS Fe2+ in octahedral coordination. The high-pressure Raman spectra evidence the Fe2+ HS-to-LS transition mainly from the abrupt shift of the P-O symmetric stretching modes to lower frequencies at similar to 76GPa, the highest pressure achieved in the Raman spectroscopic experiments. Calculations indicated that the presence of Mn-M2(2+) simply shifts the isostructural HS-to-LS transition to higher pressures compared to the triphylite Fe-M2(2+) end-member, in qualitative agreement with our experimental observations.}, language = {en} } @article{SpiekermannWilkeJahn2016, author = {Spiekermann, Georg and Wilke, Max and Jahn, Sandro}, title = {Structural and dynamical properties of supercritical H2O-SiO2 fluids studied by ab initio molecular dynamics}, series = {Chemical geology : official journal of the European Association for Geochemistry}, volume = {426}, journal = {Chemical geology : official journal of the European Association for Geochemistry}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0009-2541}, doi = {10.1016/j.chemgeo.2016.01.010}, pages = {85 -- 94}, year = {2016}, abstract = {In this study we report the structure of supercritical H2O-SiO2 fluid composed of 50 mol\% H2O and 50 mol\% SiO2 at 3000 K and 2400 K. investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 138 g/cm(3), which yields a pressure of 4.3 GPa at 3000 K and 3.6 GPa at 2400 K. Throughout the trajec[ories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH = SiOSi + H2O The calculation of the reaction constant K- [OH](2)/[H2O][O2-] is carried out on the basis of the experimentally relevant Q ' species notation and agrees well with an extrapolation of experimental data to 3000 K. After quench from 3000 K to 2400 K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H2O-silica interaction dynamics is described on the basis of the bond autocorrelation function. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} }