Hendrik Meyer

Coarse Graining of Nonbonded Inter-Particle Potentials Using Automatic Simplex Optimization to Fit Structural Properties

We implemented a coarse-graining procedure to construct mesoscopic models of complex molecules. The final aim is to obtain better results on properties depending on slow modes of the molecules. Therefore the number of particles considered in molecular dynamics simulations is reduced while conserving as many properties of the original substance as possible. We address the problem of finding nonbonded interaction parameters which reproduce structural properties from experiment or atomistic simulations. The approach consists of optimizing automatically nonbonded parameters using the simplex algorithm to fit structural properties like the radial distribution function as target functions. Moreover, any mix of structural and thermodynamic properties can be included in the target function. Different spherically symmetric inter-particle potentials are discussed. Besides demonstrating the method for Lennard-Jones liquids, it is applied to several more complex molecular liquids such as diphenyl carbonate, tetrahydrofurane, and monomers of poly(isoprene).

Formation of chain-folded structures in supercooled polymer melts

Molecular dynamics simulations of supercooled polymer melts are reported for long chains. This is the first simulation study at melt densities where the formation of chain-folded structures resembling the lamellae of polymer crystals is observed. Crystallization and subsequent melting temperatures are related linearly to the inverse lamellar thickness. Analysis of the single chain conformations in the crystal shows that most chains reenter the same lamella by tight backfolds. Simulations are performed with a mesoscopic bead-spring model including a specific angle bending potential. They demonstrate that chain stiffness alone, without an attractive inter-particle potential, is a sufficient driving force for the formation of chain-folded lamellae.

Scattering analysis of Ar-cluster beams

Abstract The formation of Arn clusters up to n = 6 in a supersonic nozzle beam expansion and the fragmentation by electron impact ionization is measured by analysing the kinematically different behaviour in a scattering experiment. The influence of dissociative scattering on the analysis is investigated.

CG-OPT: A software package for automatic force field design

Abstract The concept and details of implementation, user interface, and file formats of the Coarse-Graining OPTimization “CG-OPT” (version 2.0) software package are described. It enables the user to map a full-detailed atomistic model of a polymer system to a coarse-grained mesoscale model (roughly one bead per chemical repeat unit). The features include a control script, an implementation of the simplex algorithm and an implementation of a structure difference algorithm based on potential inversions from distribution functions. Taken together, this machinery is able to automatically modify and optimize the force field by way of user-selected potential functions for mesoscale polymer simulations.

Physisorption of water on salt surfaces

Abstract Results of ab initio calculations of water adsorption on clean NaCl(1xa00xa00) surfaces show that favorable adsorption geometries critically depend on the competition of water–surface and water–water interactions. At the most favorable adsorption site, the oxygen is near the cation with hydrogens forming hydrogen bonds with nearest anions. For monolayer adsorption, the water–water interaction is decisive for the clustering of water molecules. We also discuss a hexagonal (2×4) bilayer ice-like structure. Molecular-dynamics simulations give new insight into the dynamic stability of different adsorption geometries. Finally, some complementary results for KCl are given, in which case the interaction of oxygen and cation is weaker.

A Molecular-Dynamics Study of the Rhodopsin Chromophore Using Ultrasoft Pseudopotentials

We investigate the eect of dierent environments on the chromophore of the protein rhodopsin by using the Vienna ab initio simulation package which is based on Density Functional Theory with a plane wave basis set and the implementation of Vanderbilt’s ultrasoft pseudopotentials. We have calculated the energy dependence of 11-cis-retinal on the C11C12 double bond twist angle in the ground state and our results show that the isomerization in the ground state of the retinal chromophore is more dicult in the presence of the counter ion than without it.

AB INITIO MOLECULAR-DYNAMICS SIMULATIONS OF ADSORPTION OF DYE MOLECULES AT SURFACES

Max-Planck Institut fu¨r Polymerforschung, 55128 Mainz, GermanyE-mail: hmeyer@mpip-mainz.mpg.deY. SAKAMOTODepartment of Physics, Osaka University, Toyonaka, Osaka 560-0043, JapanE-mail: sakamoto@phys.sci.osaka-u.ac.jpJ. HAFNERInstitut fu¨r Materialphysik, Universtit¨at Wien, 1090 Wien, AustriaE-mail:Juergen.Hafner@univie.ac.atV. BUSSInstitut fu¨r Physikalische und Theoretische Chemie,Universit¨at Duisburg, 47048 Duisburg, GermanyE-mail:theobuss@uni-duisburg.de