Mark Wojcik

A molecular dynamics study of MgO(111) slabs

Constant stress - constant temperature molecular dynamics simulations are reported for 30 Angstrom thick crystalline MgO (111) slabs with 2-D periodicity in the temperature range 10-1100 K. The 10 ...

A combined molecular dynamics–ab initio study of H2 adsorption on ideal, relaxed, and temperature-reconstructed MgO(111) surfaces

2D periodic ab initio calculations have been performed for H-2 interacting with a temperature-reconstructed MgO(lll) surface, created from a molecular dynamics (MD) simulation at 300 K. The tempera ...

Pressure dependence and activation volume for the water exchange mechanism in NaCl(aq) from MD simulations

The influence of an external pressure on the solvent exchange mechanisms around the Na+ ion in a dilute NaCl(aq) solution has been investigated through MD simulations at 0 and 200 MPa. At zero pressure most of the exchange processes (≈ 75%) can be classified in terms of associative 6-7-6 mechanisms. At 200 MPa the exchange rate is 20% higher and the mechanistic processes become more involved. The activation volume for the 6-7-6 exchange process at zero pressure was calculated from the MD simulation using Voronoi polyhedra, and gives a negative sign for ΔV#. This supports the prevailing notion that a negative activation volume is indicative of an associative exchange mechanism.

MD simulations of a doped ceria surface – very large surface ion motion

Mean-square displacements (MSDs) and individual-ion square-displacements (ISDs) for the different constituents in Ca-doped CeO2(0 1 1) slabs at 300 K have been studied as a function of depth from t ...

Structure and predicted near edge x-ray absorption fine structure spectra for the surface of liquid formamide from molecular-dynamics simulation

A modification of an existing model for formamide interactions in the liquid phase makes it possible to study the bulk liquid and its free surface at conditions near the triple point, i.e., 300 K and essentially zero pressure, by (MD) molecular-dynamics simulation. Density profiles and orientation profiles in the surface region show the depth and gradual loss of the molecular ordering in the transition from bulk vapor to the bulk liquid. Near edge x-ray absorption fine structure (NEXAFS) resonance intensity spectra are computed from the orientation profiles, so that comparison should be possible with experimental depth resolved NEXAFS spectra.

Molecular dynamics simulations of an Al2O3(0001⊥, 0-10∥)/CeO2 (011⊥, 01-1∥) interface system

Constant stress, constant temperature (10 K, 300 K) molecular dynamics simulations were carried out with shell-model potentials for an infinite composite ceria-alumina slab with two free surfaces [ ...

The problem of the detaching shell in the shell model potential for oxides

Abstract Many sets of shell model parameters for oxides suffer from the defect that when the anion–cation pair is not restrained by the normal lattice forces, the ions collapse into each other. This can degrade their usefulness in the study of, for example, solid surfaces where normal lattice restraining forces are missing. We discuss adding a higher-order constraining force to the core–shell interaction in the failed models. The effects on the relative permittivities and elastic constants suggest a general reparameterization, in conjunction with standardized higher-order constraining forces, is needed. This constitutes an extension of the shell model. The ensuing models should acquire more realistic repulsive forces at short range.