SCI Publications

We present a semi-Lagrangian method for advection–diffusion and incompressible Navier–Stokes equations. The focus is on constructing stable schemes of secondorder temporal accuracy, as this is a crucial element for the successful application of semi-Lagrangian methods to turbulence simulations. We implement the method in the context of unstructured spectral/hp element discretization, which allows for efficient search-interpolation procedures as well as for illumination of the nonmonotonic behavior of the temporal (advection) error of the form: (see pdf for formula) We present numerical results that validate this error estimate for the advection–diffusion equation, and we document that such estimate is also valid for the Navier–Stokes equations at moderate or high Reynolds number. Two- and three-dimensional laminar and transitional flow simulations suggest that semi-Lagrangian schemes are more efficient than their Eulerian counterparts for high-order discretizations on nonuniform grids.

The dynamics of the initial thermal decomposition step of gas-phase α-HMX is investigated using the master equation method. Both the NO₂ fission and HONO elimination channels were considered. The structures, energies, and Hessian information along the minimum energy paths (MEP) of these two channels were calculated at the B3LYP/cc-pVDZ level of theory. Thermal rate constants at the high-pressure limit were calculated using the canonical variational transition state theory (CVT), microcanonical variational transition state theory (μVT). The pressure-dependent multichannel rate constants and the branching ratio were calculated using the master equation method. Quantum tunneling effects in the HONO elimination are included in the dynamical calculations and found to be important at low temperatures. At the high-pressure limit, the NO₂ fission channel is found to be dominant in the temperature range (500-1500 K). Both channels exhibit strong pressure dependence at high temperatures. Both reach the high-pressure limits at low temperatures. We found that the HONO elimination channel can compete with the NO₂ fission, one in the low-pressure and/or hightemperature regime.

The principal values of the ¹³C chemical-shift tensors of natural abundance biphenylene were measured at room temperature with the FIREMAT experiment. Of 18 crystallographically distinct positions (three sets of six congruent carbons each), the three primary bands have been resolved into seven single peaks and four degenerate peaks (two double, one triple, and one quadruple). Hence, eleven different chemical-shift tensors are reported. An interpretation of the data is made by comparison to carbon chemical-shift tensors in other molecules with similar chemical environments. Experimental and theoretical values based on a model of the asymmetric unit of the crystal unit cell are in good agreement.

The thermal conductivity of liquid octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) has been determined from imposed heat flux non-equilibrium molecular dynamics (NEMD) simulations using a previously published quantum chemistry-based atomistic potential. The thermal conductivity was determined in the temperature domain 550⩽T⩽800 K, which corresponds approximately to the existence limits of the liquid phase of HMX at atmospheric pressure. The NEMD predictions, which comprise the first reported values for thermal conductivity of HMX liquid, were found to be consistent with measured values for crystalline HMX. The thermal conductivity of liquid HMX was found to exhibit a much weaker temperature dependence than the shear viscosity and self-diffusion coefficients.

We have applied a new nonequilibrium molecular dynamics (NEMD) method [F. Müller-Plathe, J. Chem. Phys. 106, 6082 (1997)] previously applied to monatomic Lennard-Jones fluids in the determination of the thermal conductivity of molecular fluids. The method was modified in order to be applicable to systems with holonomic constraints. Because the method involves imposing a known heat flux it is particularly attractive for systems involving long-range and many-body interactions where calculation of the microscopic heat flux is difficult. The predicted thermal conductivities of liquid n-butane and water using the imposed-flux NEMD method were found to be in a good agreement with previous simulations and experiment.

Equilibrium molecular dynamics methods were used in conjunction with linear response theory and a recently published potential-energysurface [J. Phys. Chem. B 103, 3570 (1999)] to compute the liquid shear viscosity and self-diffusion coefficient of the high explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) over the temperature domain 550–800 K. Predicted values of the shear viscosity range from 0.0055 Pa ^*s at the highest temperature studied up to 0.45 Pa ^*s for temperatures near the melting point. The results, which represent the first publication of the shear viscosity of HMX, are found to be described by an Arrhenius rate law over the entire temperature domain studied. The apparent activation energy for the shear viscosity is found to scale with the heat of vaporization in a fashion consistent with those for a wide variety of simple nonmetallic liquids. The self-diffusion coefficient, which requires significantly shorter trajectories than the shear viscosity for accurate calculation, also exhibits an Arrhenius temperature dependence over the simulated temperature domain. This has potentially important implications for predictions of the shear viscosity at temperatures near the melting point.

On 4-5 December 1998 researchers from several universities, national laboratories, software companies, and government funding agencies met at Santa Fe, NM for the 1998 Scientific Integrated Development Environments for Knowledge, Information, and Computing Workshop. The purpose of this meeting was to summarize the state-of-the-art in the area of problem-solving environments (PSEs) for scientific and engineering computation, and to map out directions for future research in the area. This report presents some of the results from the meeting and recommends promising areas for further work. This report begins with a justification of the need for PSEs, which are also commonly called computational workbenches. Next a listing of characteristics that many PSEs share is presented, followed by a small sample listing of current systems. Design goals and future directions, with an emphasis on research issues, are outlined, followed by summary findings and conclusions.

A classical potential function for simulations of poly(vinylidene fluoride) (PVDF) based upon quantum chemistry calculations on PVDF oligomers has been developed. Quantum chemistry analysis of the geometries and conformational energies of 1,1,1,3,3-pentafluorobutane (PFB), 1,1,1,3,3,5,5,5-octofluoropentane (OFP), 2,2,4,4-tetrafluoropentane (TFP), and 2,2,4,4,6,6-hexafluoroheptane (HFH) was undertaken. In addition, an ab initio investigation of the energies of CF_4-CF₄ and CH_4-CF₄ dimers was performed. The classical potential function accurately reproduces the molecular geometries and conformational energies of the PVDF oligomers as well as intermolecular interactions between CH₄ and CF₄. To validate the force field, molecular dynamics simulations of a PVDF melts have been carried out at several temperatures. Simulation results are in good agreement with extant experimental data for PVT properties for amorphous PVDF as well as for PVDF chain dimensions.