2001

A. Violi, A.F. Sarofim, T.N. Truong.
**“Quantum mechanical study of molecular weight growth process by combination of aromatic molecules,”** In *Combustion and Flame*, Vol. 126, No. 1-2, pp. 1506--1515. July, 2001.

DOI: 10.1016/S0010-2180(01)00268-1

Formation pathways for high-molecular-mass compound growth are presented, showing why reactions between aromatic moieties are needed to explain recent experimental findings. These reactions are then analyzed by using quantum mechanical density functional methods. A sequence of chemical reactions between aromatic compounds (e.g., phenyl) and compounds containing conjugated double bonds (e.g., acenaphthylene) was studied in detail. The sequence begins with the H-abstraction from acenaphthylene to produce the corresponding radical, which then furnishes higher aromatics through either a two-step radical-molecule reaction or a direct radical-radical addition to another aromatic radical. Iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. This sequence can be repeated for the formation of high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. The aromatic radical attacks the double bond of five-membered-ring polycyclic aromatic hydrocarbons. This involves specific compounds that are exceptional soot precursors as they form resonantly stabilized radical intermediates, relieving part of the large strain in the five-membered rings by formation of linear aggregates.

S. Vyazovkin, J.S. Clawson, C.A. Wight.
**“Thermal Dissociation Kinetics of Solid and Liquid Ammonium Nitrate,”** In *Chemistry of Materials*, Vol. 13, No. 3, pp. 960--966. February, 2001.

DOI: 10.1021/cm000708c

Thermogravimetry has been used to study the kinetics of the thermal dissociation of solid and liquid ammonium nitrate. Model-fitting and model-free kinetic methods have been applied to the sets of isothermal and nonisothermal measurements to derive kinetic characteristics of the processes. The application of the model-fitting method to the isothermal data has demonstrated that both solid- and liquid-phase kinetics are characterized by a single activation energy of ∼90 kJ mol^{-1} and by the model of a contracting cylinder. A model-free isoconversional method has also been applied to isothermal and nonisothermal data and has yielded an activation energy of ∼90 kJ mol^{-1}, which is essentially independent of the extent of conversion. The obtained kinetic characteristics have been assigned to the process of dissociative sublimation/vaporization.

S. Zhang, T.N. Truong.
**“Branching Ratio and Pressure Dependent Rate Constants of Multi-Channel Unimolecular Decomposition of Gas-Phase a-HMX: An Ab Initio Dynamics Study,”** In *Journal of Physical Chemistry, A*, Vol. 105, pp. 2427--2434. 2001.

DOI: 10.1021/jp0043064

The dynamics of the initial thermal decomposition step of gas-phase α-HMX is investigated using the master equation method. Both the NO_{2} 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 (*μ*VT). 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 NO_{2} 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 NO_{2} fission, one in the low-pressure and/or hightemperature regime.

2000

D.H. Barich, A.M. Orendt, R.J. Pugmire, D.M. Grant.
**“Carbon-13 Chemical Shift Tensors in Polycyclic Aromatic Compounds. 9. Biphenylene,”** In *Journal of Physical Chemistry, A*, Vol. 104, No. 35, pp. 8290--8295. August, 2000.

DOI: 10.1021/jp001911y

The principal values of the ^{13}C 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.

D. Bedrov, G. Smith, T.D. Sewell.
**“Thermal Conductivity of Liquid Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine (HMX) From Molecular Dynamics Simulations,”** In *Chemical Physics Letters*, Vol. 324, No. 1-3, pp. 64--68. June, 2000.

DOI: 10.1016/S0009-2614(00)00559-5

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.

D. Bedrov, G.D. Smith.
**“Thermal Conductivity of Molecular Fluids from Molecular Dynamics Simulations: Application of a New Imposed-Flux Method,”** In *Journal of Chemical Physics*, Vol. 113, No. 18, pp. 8080--8084. 2000.

DOI: 10.1063/1.1312309

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.

D. Bedrov, G.D. Smith, T. Sewell.
**“Temperature-dependent shear viscosity coefficient of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX): A molecular dynamics simulation study,”** In *Journal of Chemical Physics*, Vol. 112, No. 16, pp. 7203--7208. 2000.

DOI: 10.1063/1.481285

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.

O.G. Byutner, G.D. Smith.
**“Quantum Chemistry Based Force Field for Simulations of Poly(vinylidene fluoride),”** In *Macromolecules*, Vol. 33, No. 11, pp. 4264--4270. May, 2000.

DOI: 10.1021/ma9918295

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-}CF_{4} and CH_{4-}CF_{4} 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 CH_{4} and CF_{4}. 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.

J.D. de St. Germain, J. McCorquodale, S.G. Parker, C.R. Johnson.
**“Uintah: A Massively Parallel Problem Solving Environment,”** In *Ninth IEEE International Symposium on High Performance and Distributed Computing*, IEEE, Piscataway, NJ, pp. 33--41. Nov, 2000.

T.C. Henderson, P.A. McMurtry, P.J. Smith, G.A. Voth, C.A. Wight, D.W. Pershing.
**“Simulating Accidental Fires and Explosions,”** In *Computing in Science and Engineering*, Vol. 2, No. 2, pp. 64--76. 2000.

DOI: 10.1109/5992.825750

The Center for the Simulation of Accidental Fires and Explosions (C‐SAFE) at the University of Utah is focused on providing state‐of‐the‐art, science‐based tools for the numerical simulation of accidental fires and explosions, especially within the context of handling and storage of highly flammable materials. The objective of the C‐SAFE effort is to provide a scalable, high‐performance system composed of a problem‐solving environment in which fundamental chemistry and engineering physics are fully coupled with non‐linear solvers, optimization, computational steering, visualization and experimental data verification. The availability of simulations using this system will help to better evaluate the risks and safety issues associated with fires and explosions. Our five‐year product, termed Uintah 5.0, will be validated and documented for practical application to accidents involving both hydrocarbon and energetic materials.

J. Lewis, T.D. Sewell, R. Evans, G.A. Voth.
**“Electronic Structure Calculation of the Structures and Energies of the Three Pure Polymorphic Forms of Crystalline HMX,”** In *Journal of Physical Chemistry, B*, Vol. 104, No. 5, pp. 1009--1013. January, 2000.

DOI: 10.1021/jp9926037

The molecular structures and energetic stabilities of the three pure polymorphic forms of crystalline HMX were calculated using a first-principles electronic-structure method. The computations were performed using the local density approximation in conjunction with localized “fireball” orbitals and a minimal basis set. Optimized cell parameters and molecular geometries were obtained, subject only to preservation of the experimental lattice angles and relative lattice lengths. The latter constraint was removed in some calculations for β-HMX. Within these constraints, the comparison between theory and experiment is found to be good. The structures, relative energies of the polymorphs, and bulk moduli are in general agreement with the available experimental data.

J.P. Lewis, K.R. Glaesemann, K. Van Opdorp, G.A. Voth.
**“Ab Initio Calculations of Reactive Pathways for Gas-Phase Alpha-Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine (Alpha-HMX),”** In *Physical Chemistry, A*, Vol. 104, pp. 11384--11389. 2000.

Using the BLYP and B3LYP level of density functional theory, four possible decomposition reaction pathways of HMX in the gas phase were investigated: N-NO_{2} bond dissociation, HONO elimination, C-N bond scission of the ring, and the concerted ring fission. The energetics of each of these four mechanisms are reported. Dissociation of the N-NO_{2} bond is putatively the initial mechanism of nitramine decomposition in the gas phase. Our results find the dissociation energy of this mechanism to be 41.8 kcal/mol at the BLYP level and 40.5 kcal/mol at the B3LYP level, which is comparable to experimental results. Three other mechanisms are calculated and found at the BLYP level to be energetically competitive to the nitrogennitrogen bond dissociation; however, at the B3LYP level these three other mechanisms are energetically less favorable. It is proposed that the HONO elimination and C-N bond scission reaction of the ring would be favorable in the condensed phase.

G.T. Long, S. Vyazovkin, B.A. Brems, C.A. Wight.
**“Competitive Vaporization and Decomposition of Liquid RDX,”** In *Journal of Physical Chemistry, B*, Vol. 104, No. 11, pp. 2570--2574. February, 2000.

DOI: 10.1021/jp993334n

The thermal decomposition of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) has been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Activation energies as a function of the extent of conversion, *α*, have been determined by model-free isoconversional analysis of these data. In open pans, evaporation is a prevalent process with an activation energy of ∼100 kJ mol^{-1}. Confining the system in either a pierced pan or a closed pan promotes liquid state decomposition of RDX that occurs with an activation energy of ∼200 kJ mol^{-1}, which suggests scission of an N-N bond as the primary decomposition step. In such a confined environment, gas phase decomposition is a competing channel with an activation energy estimated to be ∼140 kJ mol^{-1}. In a closed pan, RDX generates a heat release of ∼500 kJ mol^{-1} that is independent of both the heating rate, *β*, and the mass.

J.A. Nairn.
**“Fracture Mechanics of Composites with Residual Stresses, Traction-Loaded Cracks, and Imperfect Interfaces,”** In *European Structural Integrity Society: Fracture of Polymers, Composites and Adhesives*, Vol. 27, pp. 111--121. 2000.

DOI: 10.1016/S1566-1369(00)80012-6

By partitioning the total stresses in a damaged composite into either mechanical and residual stresses or into initial and pertubation stresses, it was possible to derive several exact results for the energy release rate due to crack growth. These general results automatically include the effects of residual stresses, traction-loaded cracks, and imperfect interfaces. By considering approximate solutions based on admissible stress states and admissible strain states, it was possible to derive rigorous upper and lower bounds to the energy release rate for crack growth. Two examples of using these equations are mode I fracture in adhesive double cantilever beam specimens and analysis of microcracking in composite laminates.

M. Pernice.
**“A Hybrid Multigrid Method for the Steady-State Incompressible Navier-Stokes Equations,”** In *Applied Mathematics and Computation*, Vol. 138, No. 2-3, pp. 341--353. 2000.

Multigrid methods for solving the steady-state incompressible Navier-Stokes equations require an appropriate smoother and coarse grid solution strategy to be effective. Classical pressure-correction methods, such as SIMPLE and SIMPLER, are widely used as solvers in engineering analysis codes, but can also be used as effective multigrid smoothers. An inexact Newton method preconditioned by a linear multigrid method with a pressurecorrection smoother can serve as a coarse grid solver. A hybrid nonlinear multigrid scheme based on combinations of these components is described. A standard benchmark problem is used to demonstrate the effectiveness of SIMPLER smoothing and the impact an inexact Newton coarse grid solver has on the resulting nonlinear multigrid scheme.

G.D. Smith, W. Paul, M. Monkenbusch, D. Richter.
**“A Comparison of Neutron Scattering Studies and Computer Simulations of Polymer Melts,”** In *Chemical Physics*, Vol. 261, No. 1-2, pp. 61--74. 2000.

DOI: 10.1016/S0301-0104(00)00228-7

Neutron scattering and computer simulations are powerful tools for studying structural and dynamical properties of condensed matter systems in general and of polymer melts in particular. When neutron scattering studies and quantitative atomistic molecular dynamics simulations of the same material are combined, synergy between the methods can result in exciting new insights into polymer melts not obtainable from either method separately. We present here an overview of our recent efforts to combine neutron scattering and atomistic simulations in the study of melt dynamics of polyethylene and polybutadiene. Looking at polymer segmental motion on a picosecond time scale, we show how atomistic simulations can be used to identify molecular motions giving rise to relaxation processes observed in experimental dynamic susceptibility spectra. Examining larger length and longer time scale polymer dynamics involving chain self-diffusion and overall conformational relaxation, we show how simulation results can motivate experiment and how combined results of scattering and simulation can be used to critically test theories that attempt to describe melt dynamics of short polymer chains.

P. Sutton, C.D. Hansen.
**“Accelerated Isosurface Extraction in Time-varying Fields,”** In *IEEE Trans. Vis & Comp. Graph.*, Vol. 6, No. 2, pp. 98--107. 2000.

T.N. Truong.
**“Reaction Class Transition State Theory: Hydrogen Abstraction Reactions by Hydrogen Atoms as Test Cases,”** In *Journal of Chemical Physics*, Vol. 113, No. 12, pp. 4957-4964. 2000.

DOI: 10.1063/1.1287839

We present a new method called Reaction Class Transition State Theory (RC-TST) for estimating thermal rate constants of a large number of reactions in a class. This method is based on the transition state theory framework within the reaction class approach. Thermal rate constants of a given reaction in a class relative to those of its principal reaction can be efficiently predicted from only its differential barrier height and reaction energy. Such requirements are much less than what is needed by the conventional TST method. Furthermore, we have shown that the differential energetic information can be calculated at a relatively low level of theory. No frequency calculation beyond those of the principal reaction is required for this theory. The new theory was applied to a number of hydrogen abstraction reactions. Excellent agreement with experimental data shows that the RC-TST method can be very useful in design of fundamental kinetic models of complex reactions.

T.N. Truong, D.K. Maity, T.-T.T. Truong.
**“A combined reaction class approach with integrated molecular orbital+molecular orbital (IMOMO) methodology: A practical tool for kinetic modeling,”** In *Journal of Chemical Physics*, Vol. 112, No. 1, pp. 24--30. 2000.

DOI: 10.1063/1.480558

We present a new practical computational methodology for predicting thermal rate constants of reactions involving large molecules or a large number of elementary reactions in the same class. This methodology combines the integrated molecular orbital+molecular orbital (IMOMO) approach with our recently proposed reaction class models for tunneling. With the new methodology, we show that it is possible to significantly reduce the computational cost by several orders of magnitude while compromising the accuracy in the predicted rate constants by less than 40% over a wide range of temperatures. Another important result is that the computational cost increases only slightly as the system size increases.

S. Vyazovkin, C.A. Wight.
**“Estimating Realistic Confidence Intervals for the Activation Energy Determined from Thermoanalytical Measurements,”** In *Analytical Chemistry*, Vol. 72, No. 14, pp. 3171--3175. June, 2000.

DOI: 10.1021/ac000210u

A statistical procedure is proposed for estimating realistic confidence intervals for the activation energy determined by using an advanced isoconversional method. Nine sets of five thermogravimetric measurements have been produced for the process of gassification of ammonium nitrate at five different heating rates. Independent estimates of the confidence intervals for the activation energy have been obtained from these data sets. Agreement with these independent estimates demonstrates that the proposed statistical procedure is capable of adequately estimating the actual uncertainty in the activation energy determined from a small number of measurements. The resulting averaged relative errors in the activation energy were found to be 26, 21, and 17% for three, four, and five heating rate estimates, respectively.