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Manuscript Title: ABCRATE: a program for the calculation of atom-diatom reaction rates.
Authors: B.C. Garrett, G.C. Lynch, T.C. Allison, D.G. Truhlar
Program title: ABCRATE version 10.0
Catalogue identifier: ADHL_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 109(1998)47
Programming language: Fortran.
Computer: Cray Y-MP C90, Silicon Graphics IRIS Power Challenge L, IBM RS/6000-550.
Operating system: UNICOS 8.0.3, IRIX 6.2, AIX 4.1.
RAM: 26M words
Word size: 64
Peripherals: disc.
Keywords: Molecular physics, Chemical kinetics, Reaction chemical rates, Activation energy, Stationary point Analysis, Reaction coordinate, Anharmonicity, Variational transition, State theory, Small-curvature Tunneling, Large-curvature Tunneling, Least-action Ground-state Tunneling.
Classification: 16.12.

Nature of problem:
The program calculates bimolecular atom-diatom chemical reaction rates using global or semi-global analytical potential energy surfaces.

Solution method:
Rate constants may be calculated by various versions of variational transition state theory or the unified statistical model, in either case with our without multidimensional semiclassical tunneling contributions. Rate constants may be calculated for canonical ensembles or for a special initial vibrational state with other modes thermalized. The semiclassical tunneling methods available in this version of the program are the minimum-energy-path semiclassical ground-state (MEPSAG) method, the centrifugal-dominant small-curvature semiclassical adiabatic ground- state (CD-SCSAG) method, the large curvature ground-state version-3 (LCG3) method, the least-action ground-state (LAG) method, and the microcanonical optimized multidimensional tunneling (muOMT) method. (The first four of these methods are usually abbreviated ZCT, SCT, LCT, and LAT, respectively.) The program can also be used to carry out stationary point analyses for transition states and conventional transition state theory rate constants. First the program optimizes the geometry of the reactant diatomic, the product diatomic, and the saddle point(s) (if any exist). The program is designed to handle zero to four saddle points. The reference path is found by following the path of steepest descent in a mass-scaled coordinate system from a collinear saddle point or from a pre-specified point in the asymptotic region (the latter option is for exoergic reactions with monotonically downhill reaction paths). Tunneling probabilities are calculated analytically by the Wigner method and semiclassically (six methods listed above) by numerical quadrature. The variational transition state of canonical variational theory and improved canonical variational theory is found by interpolation of data on a storage grid. Reverse reaction rates are deterimined by detailed balance.

This code is applicable only to atom-diatom reactions with a collinear reaction path. The program has PARAMETER control for the maximum number of save points at which the reaction-path information is saved; this parameter is set in the INCLUDE file abc.inc. Large-curvature tunneling is available only with the minimum energy path as the reference path, i.e., version 3.

Unusual features:
A restart option is available in which the program writes properties it has calculated to disk files. There are three different independent sets of information which can be written: the properties for the minimum energy path, the WKB eigenenergies, and the one and three-dimensional transmission probabilities for the large-curvature tunneling methods. Subsequent runs of the program can read in one or all of these files and proceed without recomputing the information that has been input. The program is distributed with a documentation file in portable, searchable ASCII text format.
The program executes in REAL*8 precision on all machines. (The code is written in double precision with generic Fortran fuction calls so it executes in REAL*8 mode on 32-bit workstations and on 64-bit workstations on which single precision is 32 bits. On Cray machines on which single precision is 64-bit words, the code should be compiled with double precision disabled so that it will execute in REAL*8 mode.)

Running time:
The running time for ABCRATE ranges from a fraction of a second to several hours depending on the input parameters selected by the user. Timings for all of the test suite runs on three different platforms are given in the ASCII manual.