Ersan Demiralp

Factors affecting molecular dynamics simulated vitreous silica structures

To obtain accurate results in molecular dynamic (MD) simulations of glasses, it is essential to chose proper force fields (FF), proper length of the simulation cell, and proper cooling cycle to generate 300 K structures from liquid. Herein we establish guidelines for these choices. We find that the MS-Q force field (FF) and the Molecular Simulations Incorporated (MSI) glass FF both lead to agreement with the radial distribution function (RDF) from small angle neutron scattering (SANS) experiments. We find that the simulation cell should contain about 3000 atoms to obtain run to run density variation less than 1% and consistency in the first two RDF peaks of 0.001 nm. A cell of 648 atoms gives run to run density variation of up to 5% and consistency in the first three RDF peaks of 0.001 nm. We find that a good compromise between accuracy and reproducibility of results and simulation time is to start with NVT dynamics at 8000 K followed by cooling to room temperature at a rate of 100 K/2 ps.

Computational Materials Chemistry at the Nanoscale

In order to illustrate how atomistic modeling is being used to determine the structure, physical, and chemical properties of materials at the nanoscale, we present here the results of molecular dynamics (MD) simulations on nanoscale assemblies of such materials as carbon nanotubes, diamond surfaces, metal alloy nanowires, and ceramics. We also include here the results of nonequilibrium MD simulations on the nanorheology of a monolayer of wear inhibitor self-assembled on two metal oxide surfaces, separated by hexadecane lubricant, and subjected to steady state shear.We also present recent developments in force fields (FF) required to describe bond breaking and phase transformations in such systems. We apply these to study of plasticity in metal alloy nanowires where we find that depending on the strain rate, the wire may deform plastically (forming twins), neck and fracture, or transition to the amorphous phase.

Prediction of new donors for organic superconductors

Abstract The donors of all known one- or two-dimensional organic superconductors are based on a core organic molecule that is either tetrathiafulvalene (denoted as TTF) or tetraselenafulvalene (denoted as TSeF) or some mixture of these two molecules. Coupling X, with appropriate acceptors, Y, leads to superconductivity. The oxidized form of X may be X + or X 2 + species in the crystal. From ab initio quantum mechanical calculations (HF/6–31G ∗∗ ), we find that all known organic superconductors involve an X that deforms to a boat structure while X + is planar. This leads to a coupling between charge transfer and the boat deformation phonon modes that we believe is responsible for the superconductivity of these materials. Based on this idea we have developed similar organic donors having the same properties and suggest that with appropriate electron acceptors they will also lead to superconductivity.

Theoretical studies on VPI-5. 3.: The MS-Q force field for aluminophosphate zeolites

Abstract Aluminophosphate zeolite is an artificial material which shows strange hydrophilicity. We have been investigating the reason by quantum mechanics, and found that the hydrophilicity of this zeolite depended on the local geometric deformation, and speculated that the site-specific hydrophilicity might be explained by differences in the stiffness of the local deformation determined by the location with the pore structure. In order to test these ideas, we construct a reliable force field based on the new MS-Q approach developed by Demiralp, Cagin, and Goddard. Our force field well reproduces the experimental structure of VPI-5.