Fragmentation Methods
To address the steep computational scaling of our most accurate theoretical methods, we have recently developed a new extrapolated fragment-based approach, termed molecules-in-molecules (MIM), for accurate energy calculations on large molecules. In this method, we use a multilevel partitioning approach coupled with electronic structure studies at multiple levels of theory to provide a hierarchical strategy for systematically improving the computed results.
In particular, we use a generalized hybrid energy expression, similar in spirit to that in the popular ONIOM methodology, that can be combined easily with any fragmentation procedure. In the current work, we explore a MIM scheme which first partitions a molecule into non-overlapping fragments and then recombines the interacting fragments to form overlapping subsystems. By including all interactions with a cheaper level of theory, the MIM approach is shown to significantly reduce the errors arising from a single level fragmentation procedure. This is a very new and active field of research, with many exciting avenues to explore, both on the application and development fronts.