Brute-force directed scans optimize freely at every point (Scheduler.run_job passes constraints=None) — flat V(φ)

Summary

A brute-force directed scan (directed_rotors: {brute_force_opt: [...]}) optimizes freely at every dihedral point instead of holding the scanned dihedral, so every point relaxes to the same minimum and the resulting V(φ) is flat (≈ zero barrier). This is silent — no error, just a wrong potential — and it is not software-specific: it reproduces on the Gaussian adapter as well as on the ASE adapter.

Root cause

Scheduler.run_job() always passes constraints=None, and no adapter derives a constraint from the torsions + dihedrals it is handed instead. So the per-point optimization that a directed scan spawns has nothing pinning the scanned dihedral, and each point optimizes to the nearest minimum.

Signature of the bug: for a directed scan, the relaxed energies across all points span ≈ 0 (a flat V(φ)), because every point collapsed to the same geometry.

Evidence

Found while running UMA (fairchem) hindered-rotor scans through the generic ASEAdapter on a cluster. With the ASE adapter made to translate torsions+dihedrals into a FixInternals constraint, the same DMH N–N torsion that had given a flat V(φ) instead held each point's dihedral to 0.000° and produced an 8.5 kJ/mol barrier. Tracing it showed the missing constraint originates upstream of any adapter, in run_job() — hence it affects Gaussian brute-force directed scans too.

Scope / workaround

PR #985 (queue-executed ASE/UMA jobs) works around this inside the ASE adapter only — it derives the constraint locally so UMA rotor scans are correct. The general fix — deriving the constraint in Scheduler.run_job() (or having each adapter honor torsions+dihedrals) so all brute-force directed scans are constrained — is out of scope for that PR and is what this issue tracks.

Suggested fix

In Scheduler.run_job() / the directed-scan spawn path, translate the job's torsions + dihedrals into the constraints argument (1-indexed atom tuples + target value) before dispatch, so every adapter receives a real constraint. Add a regression check that a brute-force directed scan on a floppy rotor produces a non-flat V(φ).

Start at Scheduler.run_job() and its directed-scan spawn path, tracing torsions and dihedrals into constraints before adapter dispatch. Compare how the Gaussian and ASE adapters receive these values. Done means a floppy-rotor brute-force directed scan has a regression check showing a non-flat V(φ), rather than energies collapsing to one minimum.