LAMMPS + MOPAC/xTB QM-MD via MDI

Driver-side (lammps_step) notes for the campaign that lets LAMMPS run MD with forces from a semiempirical QM engine (MOPAC, later xTB) over the MDI protocol.

Contents:

• Phase C – LAMMPS driver: read _model_chemistry, launch the MDI engine
  • Phase numbering
  • Scope
  • What we consume (the Phase B contract)
  • The existing MACE path, and why MOPAC is a new path
  • Proposed design
  • Decisions (resolved 2026-06-24)
  • Post-verification refinements (2026-06-24)
  • Tests
  • References
• Phase C runbook – manual end-to-end verification of QM-MD via MDI
  • Purpose
  • Prerequisites
  • Pre-flight smoke checks (cheap, isolate failures early)
  • The thin-line run
  • What to check
  • Troubleshooting
  • After a successful run
  • References