Polyrate 2023: A Computer Program for the Calculation of Chemical Reaction Rates for Polyatomics.

Polyrate 2023 is a suite of computer programs for the calculation of chemical reaction rates of polyatomic species (including atoms and diatoms as special cases) by variational transition state theory (VTST); conventional transition state theory is also supported.  Polyrate 2023 (called Polyrate for short) consists of the main program, called POLYRATE, and a set of utility codes.  

Polyrate can calculate the rate constants for both bimolecular reactions and unimolecular reactions, and it can be applied to reactions in the gas‑phase, liquid-solution phase, or solid-state and to gas‑solid‑interface reactions. Polyrate can perform variational transition state theory (VTST) calculations on gas-phase reactions with both tight and loose transition states. For tight transition states it uses the reaction-path (RP) variational transition state theory developed by Garrett and Truhlar, and for loose transition states it uses variable-reaction-coordinate (VRC) variational transition state theory developed by Georgievskii and Klippenstein. 

The RP methods used for tight transition states are conventional transition state theory, canonical variational transition state theory (CVT), and microcanonical variational transition state theory (mVT) with multidimensional semiclassical approximations for tunneling and nonclassical reflection.  The tunneling approximations available are zero-curvature tunneling (ZCT), small‑curvature tunneling (SCT), large-curvature- tunneling (LCT), and optimized multidimensional tunneling (OMT). The SCT option is the centrifugal dominant semiclassical adiabatic ground-state tunneling, and the LCT options include both LC3 and LC4 including tunneling into excited states. One may also treat specific vibrational states of selected modes with translational, rotational, and other vibrational modes treated thermally. 

For RP calculations, several options are available for reaction paths, vibrations transverse to the reaction path, and transition state dividing surfaces.  Generalized-transition-state dividing surfaces may be defined based on gradient directions in isoinertial coordinates or by the re-orientation of the dividing surface algorithm. Reaction paths may be calculated by the Euler steepest-descent, Euler stabilization, Page-McIver, or variational-reaction-path algorithms. Vibrations away from the reaction path may be defined by rectilinear (not recommended), nonredundant curvilinear, or redundant curvilinear coordinates. Vibrational frequencies may be unscaled (not recommended) or scaled.

For VRC calculations, rate constants may be calculated for canonical or microcanonical ensembles or energy-and-total-angular-momentum resolved microcanonical ensembles. VRC calculations are most appropriate for barrierless association reactions. Both single-faceted and multifaceted dividing surfaces are supported.

Pressure-dependent rate constants for elementary reactions can be computed using system-specific quantum RRK theory (SS-QRRK) with the information obtained from high-pressure-limit VTST calculation as input by using the SS-QRRK utility code. The SS-QRRK utility program is part of the Polyrate distribution, and its usage is described in a separate manual.