The CPU and memory requirements of traditional quantum chemistry (QC) methods scale poorly with system size. The many-body expansion (MBE) and other fragmentation methods have have been used to circumvent these scaling issues by splitting intractably large systems into manageable subsystems formed by combinations of monomer units. Previous work has shown that "top-down" energy screening can effectively tame the combinatorial growth of the MBE with three-body terms but we are unable to extend this approach to four-body terms in a manner that is tractable yet preserves accuracy. Instead, we have developed a new "bottom-up" algorithm for screening fragments in energy space, which accounts for the inter-fragment dependencies that prevent the use of simpler brute-force screening approaches. This strategy has been shown to reduce the total number of fragments in large water cluster isomers by up to 90% while maintaining 1 kcal/mol accuracy relative to traditional QC methods. This work lays the groundwork for performing accurate and efficient <i>ab initio</i> molecular dynamics simulations on large condensed phase systems using correlated methods.

Energy-screened many-body expansion including four-body terms

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