Christof Köhler

Parameter calibration of transition-metal elements for the spin-polarized self-consistent-charge density-functional tight-binding (DFTB) method : Sc, Ti, Fe, Co, and Ni

Recently developed parameters for five first-row transition-metal elements (M = Sc, Ti, Fe, Co, and Ni) in combination with H, C, N, and O as well as the same metal (M-M) for the spin-polarized self-consistent-charge density-functional tight-binding (DFTB) method have been calibrated. To test their performance a couple sets of compounds have been selected to represent a variety of interactions and bonding schemes that occur frequently in transition-metal containing systems. The results show that the DFTB method with the present parameters in most cases reproduces structural properties very well, but the bond energies and the relative energies of different spin states only qualitatively compared to the B3LYP/SDD+6-31G(d) density functional (DFT) results. An application to the ONIOM(DFT:DFTB) indicates that DFTB works well as the low level method for the ONIOM calculation.

Parametrization of the SCC-DFTB Method for Halogens

Parametrization of the approximative DFT method SCC-DFTB for halogen elements is presented. The new parameter set is intended to describe halogenated organic as well as inorganic molecules, and it is compatible with the established parametrization of SCC-DFTB for carbon, hydrogen, oxygen, and nitrogen. The performance of the parameter set is tested on a representative set of molecules and discussed.

Proton Conductivity of SO3H‐Functionalized Benzene–Periodic Mesoporous Organosilica

The proton conductivity of benzene-periodic mesoporous silica (PMO) materials functionalized with sulfonic acid groups is investigated using experimental and theoretical techniques. The SO(3) H functionalization of pristine benzene-PMO is realized by three different pathways based on a grafting method in which surface silanol groups and/or benzene rings are used to anchor SO(3) H groups for enhanced proton conductivity. The functionalized material is experimentally characterized using X-ray diffraction, small-angle neutron scattering, and argon adsorption isotherms. After pressing the functionalized benzene-PMOs into pellets, the proton conductivity is deduced from Bode plots of impedance spectra taken in the temperature range of 333-413 K at 100% relative humidity. Using quantum mechanical approaches for selected proton-conduction mechanisms, the free energy barriers for proton transport as well as the local water environment at the surface are calculated. These calculations indicate that different mechanisms from purely bulk water transport are important for the benzene-PMO proton conduction, in agreement with experimental data.

Kinetic aspects of the thermostatted growth of ice from supercooled water in simulations

In experiments, the growth rate of ice from supercooled water is seen to increase with the degree of supercooling, that is, the lower the temperature, the faster the crystallization takes place. In molecular dynamics simulations of the freezing process, however, the temperature is usually kept constant by means of a thermostat that artificially removes the heat released during the crystallization by scaling the velocities of the particles. This direct removal of energy from the system replaces a more realistic heat-conduction mechanism and is believed to be responsible for the curious observation that the thermostatted ice growth proceeds fastest near the melting point and more slowly at lower temperatures, which is exactly opposite to the experimental findings [M. A. Carignano, P. B. Shepson, and I. Szleifer, Mol. Phys. 103, 2957 (2005)]. This trend is explained by the diffusion and the reorientation of molecules in the liquid becoming the rate-determining steps for the crystal growth, both of which are slower at low temperatures. Yet, for a different set of simulations, a kinetic behavior analogous to the experimental finding has been reported [H. Nada and Y. Furukawa, J. Crystal Growth 283, 242 (2005)]. To clarify this apparent contradiction, we perform relatively long simulations of the TIP4P/Ice model in an extended range of temperatures. The temperature dependence of the thermostatted ice growth is seen to be more complex than was previously reported: The crystallization process is very slow close to the melting point at 270 K, where the thermodynamic driving force for the phase transition is weak. On lowering the temperature, the growth rate initially increases, but displays a maximum near 260 K. At even lower temperatures, the freezing process slows down again due to the reduced diffusivity in the liquid. The velocity of the thermostatted melting process, in contrast, shows a monotonic increase upon raising the temperature beyond the normal melting point. In this case, the effects of the increasing thermodynamic driving force and the faster diffusion at higher temperatures reinforce each other. In the context of this study, we also report data for the diffusion coefficient as a function of temperature for the water models TIP4P/Ice and TIP4P/2005.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds

We present a parametrization of a self‐consistent charge density functional‐based tight‐binding scheme (SCC‐DFTB) to describe gold‐organic hybrid systems by adding new Au‐X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold‐thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and AunSCH3 (n = 3 and 25) molecular‐sized models. The geometrical, energetic, and electronic parameters obtained at the SCC‐DFTB level for the small Au3SCH3 gold–thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold–thiolate model, Au25SCH3, the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC‐DFTB parameters are also in good agreement with DFT results.

Graphene nucleation on a surface-molten copper catalyst: quantum chemical molecular dynamics simulations

Chemical vapor deposition (CVD) growth of graphene on Cu(111) has been modeled with quantum chemical molecular dynamics (QM/MD) simulations. These simulations demonstrate at the atomic level how graphene forms on copper surfaces. In contrast to other popular catalysts, such as nickel and iron, copper is in a surface molten state throughout graphene growth at CVD-relevant temperatures, and graphene growth takes place without subsurface diffusion of carbon. Surface Cu atoms have remarkably high mobilities on the Cu(111) surface, both before and after graphene nucleation. This surface mobility drives “defect healing” processes in the nucleating graphene structure that convert defects such as pentagons and heptagons into carbon hexagons. Consequently, the graphene defects that become “kinetically trapped” using other catalysts, such as Ni and Fe, are less commonly observed in the case of Cu. We propose this mechanism to be the basis of coppers ability to form high-quality, large-domain graphene flakes.