Multi-Species Multi-Channel (MSMC) is an ab initio parallel program to calculate thermodynamic quantities (e.g., H 298 f , H (T) − H (0), S(T), and C p (T)), time-dependent species profiles, and rate coefficients as functions of temperature and pressure for complex chemical reaction systems, which consist of multiple stable species and multiple reaction channels interconnecting them. Thermodynamic properties of the species involved are calculated using statistical mechanics with molecular information from electronic structure calculations. Temperature-and pressure-dependent behaviors are rigorously characterized within the eigenpair master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework. Corrections, e.g., for hindered internal rotation and tunneling treatment, are included. With the implementation of an ultra-high precision package and rigorous matrix setup, MSMC is able to correctly mimic real behaviors of different types of chemical systems. Different eigenpair-based approaches to extract phenomenological/macroscopic rate coefficients are implemented for different applications. Moreover, a friendly and platformindependent graphical-user-interface (GUI) is provided to facilitate the use of MSMC and the pre-/postcalculation data visualization/analysis on the fly. The program can be freely downloaded at

Thermal rate constants of the CH 4 ? O 2 = CH 3 ? HO 2 reaction were calculated from first principles using both the conventional transition state theory (TST) and canonical variational TST methods with correction from the explicit hindered rotation treatment. The CCSD(T)/ aug-cc-pVTZ//BH&HLYP/aug-cc-pVDZ method was used to characterize the necessary potential energy surface along the minimum energy path. We found that the correction for hindered rotation treatment, as well as the re-crossing effects noticeably affect the rate constants of the title process. The calculated rate constants for both forward and reverse directions are expressed in the modified Arrhenius form as k CVT=HR forward ¼ 2:157  10 À18  T 2:412  exp ðÀ 25812 T Þ and k CVT=HR reverse ¼ 1:375  10 À19  T 2:183  exp ð 2032 T Þ (cm 3 molecule-1 s-1) for the temperature range of 300-2,500 K. Being in good agreement with literature data, the results provide solid basis information for the investigation of the entire alkane ? O 2 = alkyl radical ? HO 2 reaction class.

The detailed kinetic mechanism of the HOSO + O reaction, which plays a pivotal role in the atmospheric oxidation of SO, was investigated using accurate electronic structure calculations and novel statistical thermodynamic/kinetic models. Explored using the accurate composite method W1U, the detailed potential energy surface (PES) revealed that the addition of O to a HOSO radical to form the adduct (HOSO) proceeds via a transition state with a slightly positive barrier (i.e., 0.7 kcal mol at 0 K). Such a finding compromises a long-term hypothesis about this channel of being a barrierless process. Moreover, the overall reaction was found to be slightly exothermic by 1.7 kcal mol at 0 K, which is in good agreement with recent studies. On the newly-constructed PES, the temperature- and pressure-dependent behaviors of the title reaction were characterized in a wide range of conditions (T = 200-1000 K & P = 10-760 Torr) using the integrated deterministic and stochastic master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate model in which corrections for hindered internal rotation (HIR) and tunneling treatments were included. The calculated numbers were found to be in excellent agreement with literature data. The sensitivity analyses on the derived rate coefficients with respect to the ab initio input parameters (i.e., barrier height and energy transfer) were also performed to further understand the kinetic behaviors of the title reaction. The detailed kinetic mechanism, consisting of thermodynamic and kinetic data (in NASA polynomial and modified Arrhenius formats, respectively), was also provided at different T & P for further use in the modeling/simulation of any related systems.

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An integrated deterministic and stochastic model within the master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework was first used to characterize temperature- and pressure-dependent behaviors of thermal decomposition of acetic anhydride in a wide range of conditions (i.e., 300-1500 K and 0.001-100 atm). Particularly, using potential energy surface and molecular properties obtained from high-level electronic structure calculations at CCSD(T)/CBS, macroscopic thermodynamic properties and rate coefficients of the title reaction were derived with corrections for hindered internal rotation and tunneling treatments. Being in excellent agreement with the scattered experimental data, the results from deterministic and stochastic frameworks confirmed and complemented each other to reveal that the main decomposition pathway proceeds via a 6-membered-ring transition state with the 0 K barrier of 35.2 kcal·mol. This observation was further understood and confirmed by the sensitivity analysis on the time-resolved species profiles and the derived rate coefficients with respect to the ab initio barriers. Such an agreement suggests the integrated model can be confidently used for a wide range of conditions as a powerful postfacto and predictive tool in detailed chemical kinetic modeling and simulation for the title reaction and thus can be extended to complex chemical reactions.

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