At the beginning, the relationship among the GPOP components may look complicated, but, I hope, gradually you will find that they are reasonably arranged.   For example, you will find that you usually need to investigate and edit default *.mod files manually, before proceeding to the gpop3tst.   This is because, in many cases, the molecules and transition states have rotational conformer(s) which should be added as isomStates and/or internal rotation(s) which should be treated with setIntRotor.

  The relationship among the program units is schematically shown below, including the components of the other program suites, UNIMOL and SSUMES, which are the solvers of the unimolecular master-equations based on the RRKM theory.   Though not directly linked by input/output, The GPOP program can calculate the internal rotor properties (e.g., reduced rotational constants) required by the BEx1D program and the BEx1D program will be useful to determine the treatment of an internal rotor, that is, whether it should be treated as a free rotor, hindered rotor, or rotational conformers.