 The reactions involving the user specified constituents are extracted by the first AutoChem preprocessor program called Pick.This subset of reactions is then used by the second AutoChem preprocessor program RoC (rate of change) to generate the time derivatives, Jacobian, and Hessian.Once the two preprocessor programs have run to completion all the Fortran 90 code has been generated that is necessary for modeling and assimilating the kinetic processes.