Ken Flurchick

Thermodynamic properties and phase transitions in CO2 molecular clusters

The thermodynamic properties of (CO2) N molecular aggregates of size 2 ⩽ N ⩽ 13 have been investigated. These crystallites exhibit well defined orientational order–disorder rotational transitions accompanied by a structural transition into a plastic crystallite phase. In addition, they exhibit melting and disassociation transitions. It is shown that the interpretation of experimental data, based upon dimer properties, depends crucially on these results. Equilibrium structures and orientations are also given.

Visualizing properties of atomic and molecular systems.

In this article scientific visualization techniques that allow for the interactive computation and display of molecular structures and properties are presented. The electronic properties are computed and displayed using the application visualization system (AVS). By combining the visualization techniques of AVS with an orbital description of molecular systems, a better understanding of the electronic behavior of the systems can be achieved. Molecular properties such as the electrostatic potential and chemical reactivity index (the Fukui function) are computed and mapped onto isodensity displays of the electron density of the system.

Integrating symbolic and numeric techniques in atomic physics

The article describes ISNAP, a program for calculating atomic properties that uses an integrated symbolic and numerical approach for arbitrary excitations from closed-shell atoms. This program generates transition matrix elements and energy formulas up to third-order perturbation via the symbolic programming language Mathematica.

Investigation of artifacts due to periodic boundary conditions

Periodic boundary conditions are routinely employed in molecular dynamics simulations to simulate bulk material and remove surface effects. However, the effect of periodic boundary conditions on simulation results is not well understood. In this paper three distinct approaches are explored in an effort to determine whether artifacts exist due to the introduction of artificial periodicity. The techniques of group theoretical statistical mechanics are applied to provide tests for artifacts due to the cubic symmetry introduced. Autocorrelation functions and generalized squared displacements for large and small system sizes are compared for artifacts and finally transport coefficients for very low density systems are compared with accepted theoretical results. None of these methods show signs of numerically significant artifacts.

hybrid molecular dynamics : an approach to low density simulations

Abstract A new molecular dynamics technique is described which addresses the special problems intrinsic to low density simulations, particularly fluids in the gas phase. This technique incorporates aspects of both the conventional hard spheres and continouos potential molecular dynamics algorithms. The difficulties inherent in low density simulations are discussed and the synthesis of existing molecular dynamics techniques into a new, hybrid algorithm is described in detail, including optimization and vectorization modifications. Finally, benchmarks for the new code are reported which show a speedup of up to a factor of 7 over a traditional molecular dynamics program.

THE USE OF THE GRIMME DFT POTENTIAL FOR DESCRIPTION OF VAN DER WAALS BONDED MOLECULAR CRYSTALS

A recent density‐functional theory (DFT) potential by Grimme has been proposed for describing long‐range dispersion corrections. This potential has been implemented into the CRYSTAL06 program and tested for a variety of molecular crystals including urea and pentaerythritol (PE). Elastic constants, ambient volumes, and vibrational frequencies are calculated with this potential and compared with those calculated using other DFT potentials.