  With the development of low order scaling methods for performing Kohn-Sham
Density Functional Theory, it is now possible to perform fully quantum
mechanical calculations of systems containing tens of thousands of atoms.
However, with an increase in the size of system treated comes an increase in
complexity, making it challenging to analyze such large systems and determine
the cause of emergent properties. To address this issue, in this paper we
present a systematic complexity reduction methodology which can break down
large systems into their constituent fragments, and quantify inter-fragment
interactions. The methodology proposed here requires no a priori information or
user interaction, allowing a single workflow to be automatically applied to any
system of interest. We apply this approach to a variety of different systems,
and show how it allows for the derivation of new system descriptors, the design
of QM/MM partitioning schemes, and the novel application of graph metrics to
molecules and materials.
