2026-07-15 Frequencies sub-step and the <HESSIAN MDI command

Add an ORCA Frequencies sub-step – the Hessian, harmonic vibrational frequencies, IR intensities, and thermochemistry via ORCA’s analytic (AnFreq) or numerical (NumFreq) second derivatives – and a custom ``<HESSIAN`` MDI command that serves the analytic Cartesian Hessian over a warm MDI connection.

This is the ORCA end of a cross-repo Hessian-over-MDI effort for the MLFF reference-data campaign: a driver that needs a Hessian gets an analytic one straight from the resident engine when the method supports it, and finite-differences the forces otherwise. The first consumer is the normal_mode_sampling_step plug-in, which reaches the engine through seamm_mdi.MDIEngine. The science spec lives in the ~/Sites lab notebook (the water → electrolyte MLFF training-set plan, decision #2, “Internal sampling = Wigner normal-mode sampling”); this page is the ORCA implementation record.

Released in orca_step 2026.7.15 (the feature) with follow-up polish in 2026.7.16.

Contents:

• Frequencies sub-step and <HESSIAN – scope and design
  • Goal
  • The Frequencies sub-step
  • The <HESSIAN MDI command
  • Truthful capability advertising (the key design decision)
  • Cross-repo pieces (all released 2026-07-15/16)
  • Back-pointers