seamm_mbe

Many-body expansion (MBE) corrections for periodic cells and clusters.

seamm_mbe estimates high-level energies, forces and stress for a system the high-level method can't treat directly. It takes a cheap calculation on the whole system and adds many-body increments of [high − low] computed on small isolated fragments: monomers, selected pairs and triples, general in the order.

The library does the bookkeeping and runs no calculations:

• molecule typing by formula and bond-graph topology, with charges, for water, the carbonates (EC, FEC, DMC, EMC), Li⁺, BF₄⁻, PF₆⁻, common monatomic ions, and any other molecule;
• fragment enumeration under periodic boundary conditions, with canonical keys and per-type-pair cutoffs;
• the "mixed" assignment of each increment to a periodic or a molecular low level;
• the increment algebra for energy, forces and the origin-independent virial;
• the assembled labels, with energy offsets, atomic and molecular pressure and QC.

The mbe_step SEAMM plug-in runs the fragment calculations through the Model Chemistry batch contract.

It generalizes the prototype that labelled 152 periodic water cells for MLFF training (TinkerCliffs, 2026-09-29/30), and reproduces that prototype's pilot frame from the stored fragment results (tests/data/).

The design is in MBE_correction_step_design.rst (SEAMM workspace). The campaign notes are in docs/developer_guide/campaigns/2026-10-03/.

Copyright

Copyright (c) 2026, Paul Saxe

Acknowledgements

Project based on the Computational Molecular Science Python Cookiecutter version 1.11.