Reactant combination refers to the method used for combining reactant molecules.

Users can select from two methods:

1. Sequential
   combines reactants in a pairwise manner.

2. Combinatorial
   combines all reactants of the input files.

Considering two input files with two and three reactant molecules, the following reactions are possible:

In Sequential mode, the first molecules of the two inputs are paired, then the second molecules are paired. In the end, the second molecule in the first file is paired with the third molecule in the second file, processing three reactions altogether. Generally, the number of performed reactions equals the number of molecules in the bigger input file (having more compounds).

In Combinatorial mode, every molecule in the first file is paired with every molecule in the second and altogether six reactions are processed. The number of possible reactions equals the number of molecules in input file1 multiplied by the number of molecules in input file2.