Multiple-well, multiple-path unimolecular reaction systems. I. MultiWell computer program suite

Unimolecular reaction systems in which multiple isomers undergo simultaneou s reactions via multiple decomposition reactions and multiple isomerization reactions are of fundamental interest in chemical kinetics. The computer p rogram suite described here can be used to treat such coupled systems, incl uding the effects of collisional energy transfer (weak collisions). The pro gram suite consists of MultiWell, which solves the internal energy master e quation for complex unimolecular reactions systems; DenSum, which calculate s sums and densities of states by an exact-count method; Mominert, which ca lculates external principal moments of inertia and internal rotation reduce d moments of inertia; and Thermo, which calculates equilibrium constants an d other thermodynamics quantities. MultiWell utilizes a hybrid master equat ion approach. which performs like an energy-grained master equation at low energies and a continuum master equation in the vibrational quasicontinuum. An adaptation of Gillespie's exact stochastic method is used for the solut ion. The codes are designed for ease of use. Details are presented of vario us methods for treating weak collisions with virtually any desired collisio n step-size distribution and for utilizing RRKM theory for specific unimole cular rate constants. (C) 2001 John Wiley & Sons. Inc.