AutoChem is a suite of Fortran 90 computer programs for the modeling of kinetic reaction systems. AutoChem performs automatic code generation, symbolic differentiation, analysis, and documentation. It produces a documented stand-alone system for the modeling and assimilation of atmospheric chemistry. Given databases of chemical reactions and a list of constituents defined by the user, AutoChem automatically does the following:

  1. Selects the subset of reactions that involve a user-defined list of constituents and automatically prepares a document listing the reactions;
  2. Constructs the ordinary differential equations (ODEs) that describe the reactions as functions of time and prepares a document containing the ODEs;
  3. Symbolically differentiates the time derivatives to obtain the Jacobian and prepares a document containing the Jacobian;
  4. Symbolically differentiates the Jacobian to obtain the Hessian and prepares a document containing the Hessian; and
  5. Writes all the required Fortran 90 code and datafiles for a stand-alone chemical modeling and assimilation system (implementation of steps 1 through 5).

Typically, the time taken for steps 1 through 5 is about 3 seconds. The modeling system includes diagnostic components that automatically analyze each ODE at run time, the relative importance of each term, time scales, and other attributes of the model.

This program was written by David John Lary of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at under the Software category. GSC-14862-1

NASA Tech Briefs Magazine

This article first appeared in the November, 2005 issue of NASA Tech Briefs Magazine.

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