rgsaddle

Band, min-mode, and IRC mechanics over rgmin steppers.

Two seams, stepping at both. The inner seam is rgmin's Solver: one optimizer step over an assembled band force; this crate defines no optimizer of its own. The outer seam is BandSession::step: assemble NEB forces on the caller's surface, take one solver step, report. No run-to-completion contract exists; hosts own the loop and interleave their policy (trust regions, drift budgets, acquisition, hybrid MMF) between steps. run is a convenience loop over step and nothing more.

Force assembly ports eOn's NEB mechanics with identical branch structure: tangents (Mills-Jonsson-Schenter simple, Henkelman-Jonsson improved), springs (uniform, energy-weighted, Onsager-Machlup), projections (plain elastic band, NEB, doubly nudged), and the climbing-image force with the eOn trigger rule.

Positions are unwrapped Cartesian. Minimum-image differences go through linkcell. Frames come from readcon-core (CON), readcon-db (corpus), and readcon-chemfiles (foreign trajectories, chemfiles feature on readcon-core). Cutoff neighbour lists, when a surface needs them, are vesin.

BandSession::reset is the model-update boundary: quasi-Newton history taken on one surface epoch must not survive onto the next.

IrcSession owns both Gonzalez--Schlegel/Sella and Morokuma predictor--corrector roll-down. Its atomistic constructor uses repeated mass weights and the rigid quotient; IrcSession::new_euclidean runs the same mechanics on an arbitrary-dimensional Euclidean surface.

C hosts include include/rgsaddle.h. C++ hosts include include/rgsaddle/session.hpp, the same hourglass wrap rgmin ships as xts/optimize.hpp. Sessions stay in Rust; the header is RAII over the C ABI.

Narrative docs live in docs/orgmode/ (Diataxis). The one tutorial is MEP roll-down from a first-order saddle. C and C++ ABI: docs/orgmode/howto/c-abi.org. Export with emacs --script docs/export.el from docs/; Sphinx (Shibuya) reads docs/source/. The mark is in branding/logo/.

MIT. Build and test on the remote builder, not a laptop.