Pressure-Dependent Kinetics of Initial Reactions in Iso-octane Pyrolysis

High-temperature thermal decomposition of iso-octane based on reactive molecular dynamics simulations

Branched alkanes are the major components of endothermic fuels used for advanced aircrafts. Reactive molecular dynamics (RMD) simulations are carried out to explore the detailed kinetic mechanism for the thermal decomposition of iso-octane widely used as the primary reference fuel of branched alkanes. The RMD calculations indicate that the initial decomposition mechanism of iso-octane is mainly through two pathways: (1) the C - C bond cleavage to produce smaller hydrocarbon radicals and (2) the hydrogen-abstraction reactions by small radicals including •H and •CH₃. Most of the alkenes which are associated with the endothermic capacities in the iso-octane pyrolysis are produced from the C-C β-scission reactions of alkyl radicals. Propylene and ethylene are observed to be formed in large amounts. Kinetic parameters with the activation energy of 52.1 kcal mol^-1 and pre-exponential factor of 7.2 × 10¹⁴ s^-1, based on the first-order kinetic analysis, are in good agreement with previous work.

Reduced scaling formulation of CASPT2 analytical gradients using the supporting subspace method

We present a reduced scaling and exact reformulation of state specific complete active space second-order perturbation (CASPT2) analytical gradients in terms of the MP2 and Fock derivatives using the supporting subspace method. This work follows naturally from the supporting subspace formulation of the CASPT2 energy in terms of the MP2 energy using dressed orbitals and Fock builds. For a given active space configuration, the terms corresponding to the MP2-gradient can be evaluated with O(N⁵) operations, while the rest of the calculations can be computed with O(N³) operations using Fock builds, Fock gradients, and linear algebra. When tensor-hyper-contraction is applied simultaneously, the computational cost can be further reduced to O(N⁴) for a fixed active space size. The new formulation enables efficient implementation of CASPT2 analytical gradients by leveraging the existing graphical processing unit (GPU)-based MP2 and Fock routines. We present benchmark results that demonstrate the accuracy and performance of the new method. Example applications of the new method in ab initio molecular dynamics simulation and constrained geometry optimization are given.

Potential Energy Surface for Large Barrierless Reaction Systems: Application to the Kinetic Calculations of the Dissociation of Alkanes and the Reverse Recombination Reactions

The isodesmic reaction method is applied to calculate the potential energy surface (PES) along the reaction coordinates and the rate constants of the barrierless reactions for unimolecular dissociation reactions of alkanes to form two alkyl radicals and their reverse recombination reactions. The reaction class is divided into 10 subclasses depending upon the type of carbon atoms in the reaction centers. A correction scheme based on isodesmic reaction theory is proposed to correct the PESs at UB3LYP/6-31+G(d,p) level. To validate the accuracy of this scheme, a comparison of the PESs at B3LYP level and the corrected PESs with the PESs at CASPT2/aug-cc-pVTZ level is performed for 13 representative reactions, and it is found that the deviations of the PESs at B3LYP level are up to 35.18 kcal/mol and are reduced to within 2 kcal/mol after correction, indicating that the PESs for barrierless reactions in a subclass can be calculated meaningfully accurately at a low level of ab initio method using our correction scheme. High-pressure limit rate constants and pressure dependent rate constants of these reactions are calculated based on their corrected PESs and the results show the pressure dependence of the rate constants cannot be ignored, especially at high temperatures. Furthermore, the impact of molecular size on the pressure-dependent rate constants of decomposition reactions of alkanes and their reverse reactions has been studied. The present work provides an effective method to generate meaningfully accurate PESs for large molecular system.

Theoretical and kinetic study of the reaction of C2H3 + HO2 on the C2H3O2H potential energy surface

We systematically investigate the C₂H₃ + HO₂ reaction combined with conventional transition state theory, variable reaction coordinate transition state theory and Rice–Ramsberger–Kassel–Marcus/master-equation theory.