ABCRATE: A program for the calculation of atom-diatom reaction rates Original Research Article

ABCRATE is a computer program for the calculation of atom-diatom chemical reaction rates for systems with collinear-dominated dynamics. The dynamical methods used are conventional or generalized transition state theory (GTST) and multidimensional semiclassical approximations for tunneling and nonclassical reflection. The GTST methods included in this version of the program are the canonical and improved canonical variational transition state theory (VTST) and the canonical unified statistical (CUS) method. Rate constants may be calculated for canonical ensembles or for specific vibrational states of selected modes with translational, rotational, and other vibrational modes treated thermally. The potential energy surface required by the program may be a global or semiglobal analytic function. The reaction path is calculated as the path of steepest descent in mass-scaled coordinates from a collinear saddle point, and vibrations transverse to the reaction path are treated by curvilinear internal coordinates. The vibrational modes are quantized, and anharmonicity may be included by various options, including the WKB approximation for bond stretches and the centrifugal oscillator approximation through quartic terms for the curvilinear bend coordinate. Tunneling probabilities are calculated by a variety of semiclassical methods, in particular zero-curvature tunneling (ZCT), small-curvature tunneling (SCT), large-curvature tunneling (LCT), least-action tunneling (LAT), and the microcanonical optimized multidimensional tunneling (μOMT) methods.