TUMME 2023: An Open-Source Master Equation Program for Chemical Kinetics

TUMME (Tsinghua University Minnesota Master Equation solver) is a computer program for setting up and solving master equations for chemical kinetics of unimolecular and bimolecular reactions. The master equation is discretized in terms of reactant energy bins, and rate constants are calculated from chemically significant eigenmodes. TUMME has interfaces to Gaussian, Polyrate, and MSTor output files that allow the master equation code to provide the microcanonical flux coefficients needed for the kernel of the master equation as calculated by conventional transition state theory (TST), variational transition state theory (VTST) with various tunneling methods, or multi-structural or multi-path VTST (MS-VTST/SCT or MP-VTST) with various tunneling methods. The tunneling methods supported include small-curvature tunneling (SCT), large-curvature tunneling (LCT), and microcanonical optimized multidimensional tunneling (µOMT). For mechanisms involving only unimolecular isomerization (no bimolecular pairs), TUMME 2023 solves a conservative master equation for both rate constants and time-dependent energy-bin populations. For mechanisms involving bimolecular pairs, TUMME 2023 can solve two kinds of master equation: (i) a nonconservative master equation for calculating rate constants of bimolecular reactions and (ii) a conservative master equation that includes bimolecular association in the transition matrix and that can be used for calculating the time evolution of the concentration of a pseudo-first-order bimolecular reactant. TUMME is written in double precision with Python 3; quadruple and octuple precision is also available for some subtasks in C++. The Python code can run in serial or parallel (MP or MPI), and C++ code can run on a single processor or on multiple processors with OpenMP. The program includes a manual and a tutorial.