SEAMM MBE Plug-in

A SEAMM plug-in for many-body expansion (MBE) corrections of periodic cells and clusters

• Free software: BSD-3-Clause

• Documentation: https://molssi-seamm.github.io/mbe_step/index.html

• Code: https://github.com/molssi-seamm/mbe_step

History

2026.10.10 – Ion shells

• ‘Ion shells’ Li+ first shell makes each Li⁺ and the molecules in its first shell one unit of the expansion, rebuilt for every configuration. Around Li⁺ the per-molecule expansion converges slowly; with the shell as one unit it converges as for neutral molecules. ‘Shell cutoffs’ (default Li-O and Li-F 2.6 Å) say who joins, a molecule near two ions joins the nearer one, and ‘Most molecules in a shell’ (default 5) caps crowded shells. ‘Fragments with a shell’ pairs selects a shell’s pairs but not its triples, which hold most of the cost. The job output lists the shells per configuration, and the increments file lists each unit’s molecules. Shells need a contact criterion; the dialog offers only those when shells are on.

• The energy offsets, the charges and the molecular virial stay per molecule. Counterpoise is not available with ion shells yet (hidden in the dialog, refused at run time).

• Requires seamm-mbe 2026.10.10.

2026.10.7 – Energies on the DfE0 scale by default

• The new default for ‘energy offsets’, “DfE0 (from the thermochemistry database)”, puts the labels on the energy-of-formation scale (DfE0, at 0 K) that the code steps use: each molecule type’s offset comes from the atomic reference energies at the high level in SEAMM’s thermochemistry database. The step prints the offsets it used, and stops before any calculation if the database has no atoms at the high level. ‘none’ and offsets given by hand work as before; existing flowcharts keep the value they were saved with.

• Requires seamm-thermochemistry 2026.9.28.

2026.10.6.3 – The molecular pressure by body order

• The summary and the results give the many-body correction’s share of the molecular pressure, in total and by body order, alongside the atomic one. The molecular breakdown is the one to compare with the virial errors of a cluster validation; the monomers’ share of it is zero.

• Requires seamm-mbe 2026.10.6.1.

2026.10.6.2 – A failed level stops the others

• If a level fails outright (an error, or every one of its calculations failing), the other levels are stopped at once instead of being left to run for hours: their queued and running calculations are cancelled. A rerun keeps everything already finished and submits the rest afresh.

• Requires seamm-exec 2026.10.6.2.

2026.10.6.1 – The levels run at the same time

• The levels that run as batch tasks now run at the same time, with the whole cell and the periodic fragments started first. Their long calculations overlap the molecular ones instead of following them, roughly halving the time a frame takes. Levels that use an MDI engine still run one at a time.

2026.10.6 – A low level of their own for the triples

• The new ‘triples low level’ gives the triples’ increments their own molecular low level, e.g. r2SCAN-D4 at def2-TZVPPD with the molecular low level at def2-QZVPPD for the pairs. Each triple’s increment is built entirely at that level, so the triples’ monomers and pairs are computed at both levels. It is shown only when there are triples; by default it is the molecular low level. A triples’ low level that is the molecular low level under another name is refused.

• The increments file records each increment’s low level.

• The user guide explains when to use the periodic low level for the monomers only.

• Requires seamm-mbe 2026.10.6.

2026.10.5 – A high level of their own for the triples; the increments in a file

• The new ‘triples high level’ gives the triples’ increments their own high level, e.g. revDSD at def2-TZVPPD with the high level at def2-QZVPPD for the monomers and pairs. Each triple’s increment is built entirely at that level, so the triples’ monomers and pairs are computed at both levels. It is shown only when there are triples; by default it is the high level. A triples’ level that is the high level under another name is refused, since every monomer and pair would be computed twice.

• Each configuration’s increments are written to increments_c<configuration id>.json in the step’s directory, for later analysis: each fragment’s molecules, images, pair distances, levels, energy and forces, with triples marked “chain” or “closed”.

• Requires seamm-mbe 2026.10.5.

2026.10.4.1 – Pairwise counterpoise corrects the high level only by default

• ‘pairwise’ counterpoise now corrects each pair at the high level only. The new choice ‘pairwise, both levels’ also corrects the molecular low level, as ‘pairwise’ did before. In tests on Li⁺/BF₄⁻/water/EC clusters the r2SCAN-D4 correction was tiny where it could be computed, and its ghost calculations sometimes ended at spurious energies, while the revDSD correction was 3–4 kJ/mol per molecule.

• The user guide notes that ORCA’s ghost-atom gradients are unreliable for double hybrids such as revDSD, so counterpoise-corrected forces from them need caution.

2026.10.4 – Requires seamm-exec 2026.10.3.2 for running on a cluster

• Requires seamm-exec 2026.10.3.2: each bundle of calculations sent to a cluster’s queue now runs on one node, a calculation runs inside the job itself only when it fits there, and bundles get a time limit from their calculations’ estimated cost. Before, a VASP or ORCA calculation could be spread over several nodes and fail, or be cut off by the queue’s default time limit.

• Checked end to end on TinkerCliffs: the 64-water pilot frame reproduces the prototype’s labels (energy to 0.001 meV, forces to 0.2 meV/Å RMS, pressures to about 1 atm).

2026.10.3 – Initial release: many-body expansion (MBE) corrections

• A step that estimates high-level energies, forces and, for periodic cells, the stress of a cell or a large cluster: a low-level calculation of the whole system plus [high - low] corrections computed on monomers, pairs and triples of molecules, using the seamm_mbe library.

• The levels are model chemistries: the high level, a molecular low level, and optionally a periodic low level (e.g. VASP) for the fragments of periodic cells, run as batches of calculations through seamm_exec.

• Pair and triple cutoffs, a rule for which triples to include, and a check that the cell is large enough for the chosen cutoffs.

• For a periodic low level, the fragments are placed on the cell’s FFT grid so that the grid errors cancel between the cell and its fragments.

• Optional pairwise counterpoise correction of the basis-set superposition error.

• Optional energy offsets per molecule type, and the results stored as properties and written to an extended XYZ file for training machine-learned force fields.