SCI Publications

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.

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.

Kinetics of the N+H₂↔NH+Hreaction have been studied using a direct ab initio dynamics method. Potential energy surface for low electronic states have been explored at the QCISD/cc-pVDZ level of theory. We found the ground-statereaction is N(⁴S)+H₂→NH(³Σ⁻)+H. Thermal rate constants for this reaction were calculated using the microcanonical variational transition state theory.Reaction path information was calculated at the QCISD/cc-pVDZ level of theory. Energies along the minimum energy path (MEP) were then refined at the QCISD(TQ)/cc-pVTZ level of theory. The forward and reverse barriers of the ground-statereaction are predicted to be 29.60 and 0.53 kcal/mol, respectively. The calculated rate constants for both forward and reverse reactions are in good agreement with available experimental data. They can be expressed as k(T)=2.33×10¹⁴ exp(-30.83 (kcal/mol)/RT) cm³mol⁻¹ s⁻¹ for the forward reaction and k(T)=5.55×10⁸T^1.44 exp(−0.78(kcal/mol)/RT) cm³ mol⁻¹ s⁻¹ for the reverse reaction in the temperature range 400–2500 K.

The NO₂ fission reaction of gas phase α-HMX has been studied using a direct ab initio method within the framework of microcanonical variational transition state theory (μVT). The potential energy calculations were calculated using the hybrid nonlocal B3LYP density functional theory with the cc-pVDZ basis set. The calculated results show that the potential energy of breaking the axial NO₂ groups is lower than that of breaking the equatorial NO₂ groups. No traditional transition state was found along the reaction path. Microcanonical rate constants calculation shows the variational transition state varies from 2.0 to 3.5 Å of the breaking N−N bond length as a function of the excess energy. The μVT method was used for thermal rate constants calculation over a temperature range from 250 to 2000 K. The fitted Arrhenius expression from the calculated data is k(T) = 1.66 × 10¹⁵ exp(−18748K/T) s^-1, which is in good agreement with the experimental data at low temperatures.

Triangular meshes are widely used as primary representation of surface models for networked gaming and for complex interactive design in manufacturing. Accurate triangulation of a surface with sharp features (highly varying curvatures, holes) may require an extremely large number of triangles. Fast transmission of such large triangle meshes is critical to many applications that interactively manipulate geometric models in remote networked environments. The need for a succinct representation is therefore not only to reduce static storage requirements, but also to consume less network bandwidth and thus reduce the transmission time.

In this paper we address the problem of defining a space efficient encoding scheme for both lossless and error-bounded lossy compression of triangular meshes that is robust enough to handle directly arbitrary sets of triangles including non-orientable meshes, non-manifold meshes and even non-mesh cases. The compression is achieved by capturing the redundant information in both the topology (connectivity) and geometry with possibly property attributes. Example models and results are also reported.