  The construction of a reaction network containing all relevant intermediates
and elementary reactions is necessary for the accurate description of chemical
processes. In the case of a complex chemical reaction (involving, for instance,
many reactants or highly reactive species), the size of such network may grow
rapidly. Here, we present a computational protocol that constructs such
reaction networks in a fully automated fashion steered in an intuitive,
graph-based fashion through a single graphical user interface. Starting from a
set of initial reagents new intermediates are explored through intra- and
intermolecular reactions of already explored intermediates or new reactants
presented to the network. This is done by assembling reactive complexes based
on heuristic rules derived from conceptual electronic-structure theory and
exploring the corresponding approximate reaction path. A subsequent path
refinement leads to a minimum-energy path which connects the new intermediate
to the existing ones to form a connected reaction network. Tree traversal
algorithms are then employed to detect reaction channels and catalytic cycles.
We apply our protocol to the formose reaction to study different pathways of
sugar formation and to rationalize its autocatalytic nature.
