# rgsaddle
<p align="center">
<img src="branding/logo/rgsaddle-logo-light.svg" width="420" alt="rgsaddle">
</p>
Band, min-mode, and IRC mechanics over
[rgmin](https://github.com/OmniPotentRPC/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](https://github.com/d-SEAMS/linkcell). Frames come
from [readcon-core](https://github.com/lode-org/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](https://github.com/Luthaf/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`](include/rgsaddle.h). C++ hosts
include [`include/rgsaddle/session.hpp`](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/`](docs/orgmode/index.org)
(Diataxis). The one tutorial is MEP roll-down from a first-order
saddle. C and C++ ABI: [`docs/orgmode/howto/c-abi.org`](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/`](branding/logo/README.md).
MIT. Build and test on the remote builder, not a laptop.