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Abstract The open‐source statistical mechanics software described here, Arkane–Automated Reaction Kinetics and Network Exploration–facilitates computations of thermodynamic properties of chemical species, high‐pressure limit reaction rate coefficients, and pressure‐dependent rate coefficient over multi‐well molecular potential energy surfaces (PES) including the effects of collisional energy transfer on phenomenological kinetics. Arkane can use estimates to fill in information for molecules or reactions where quantum chemistry information is missing. The software solves the internal energy master equation for complex unimolecular reaction systems. Inputs to the software include converged electronic structure computations performed by the user using a variety of supported software packages (Gaussian, Molpro, Orca,more » TeraChem, Q‐Chem, Psi4). The software outputs high‐pressure limit rate coefficients and pressure‐dependent phenomenological rate coefficients, as well as computed thermodynamic properties (enthalpy, entropy, and constant pressure heat capacity) with added energy corrections. Some of the key features of Arkane include treatment of 1D, 2D or ND hindered internal rotation modes, treatment of free internal rotation modes, quantum tunneling effect consideration, transition state theory (TST) and Rice‐Ramsperger‐Kassel‐Marcus (RRKM) rate coefficient computations, master equation solution with four implemented methods, inverse‐Laplace transform of high‐pressure limit rate coefficients into the energy domain, energy corrections based on bond‐additivity or isodesmic reactions, automated and efficient PES exploration, and PES sensitivity analysis. The present work describes the design of Arkane, how it should be used, and refers to the theory that it employs. Arkane is distributed via the RMG‐Py software suite ( https://github.com/ReactionMechanismGenerator/RMG‐Py ).« less

The low-temperature auto-ignition of fuels is a complex process, occurring in multiple stages with distinct chemical processes governing each stage. The conversion from alkyl radical to chain branching products, which occurs through successive O₂ additions followed by thermal decomposition of the products, is at the core of the auto-ignition process. Our detailed understanding of this central process continues to evolve, with recent theoretical kinetics studies providing a particularly comprehensive description of the radical oxidation process for propane. Here, we employ this improved description in a detailed numerical and analytical exploration of the first-stage ignition delay for low-temperature auto-ignition of propane,more » which may be considered as a prototype for larger alkane fuels. The traditional first-stage of ignition can be divided into two stages (stage-1A and stage-1B). During stage-1A, the concentration of radicals grows exponentially, and both OH and HO₂ are important in the consumption of the fuel and generation of alkyl radicals. Stage-1A ends when the concentration of HO₂ is sufficiently high that the chain-terminating bimolecular reaction HO₂ + HO₂ becomes competitive with other HO₂ reactions including HO₂ + fuel, thus slowing the HO₂ concentration rise such that it is no longer a key contributor to fuel consumption. During stage-1B, increasing temperature and growing side reactions with secondary chemistry reduce the positive feedback and the concentrations of ketohydroperoxide species stop growing exponentially. The end of this stage is associated with the maximum in ketohydroperoxide, after which it is depleted. We present simple analytical approximations for the time it takes to complete these two sub-stages. These expressions clarify which rate constants control first-stage ignition, and they quantify how the ignition is influenced by mixture composition, temperature and pressure. Furthermore, the analysis is extended to longer alkane fuels and is shown to provide fairly reliable predictions of the first-stage ignition delay.« less