ORCA Backend

ORCA is a quantum-chemistry backend available through PyAR for calculations where you want a higher-level electronic-structure method than xTB or a machine-learning potential.

When to use ORCA

Use orca when:

  • ORCA is installed and available on your system

  • you want DFT-style optimisation or refinement

  • you want to refine structures generated by cheaper PyAR searches

  • you have validated a small test calculation with your chosen method and basis

Example command

Reaction search using the registered ORCA energy-gradient route:

pyar-cli react A.xyz B.xyz -N 8 -gmin 100 -gmax 1000 --software orca --method B3LYP --basis def2-SVP

Practical notes

  • ORCA must be installed separately and available to PyAR.

  • Use a small test molecule first to confirm that the executable, method, basis, charge, multiplicity, and gradient output are handled correctly on your machine.

  • ORCA calculations are usually much slower than xTB or ML-potential runs. For exploration, it is often better to generate candidates with xTB/AIMNet2 and then refine selected structures with ORCA.

AFIR/geomeTRIC status

orca is wired as a registered energy-gradient provider for the geomeTRIC-backed AFIR reaction route. This means PyAR can construct a combined ORCA energy/gradient + AFIR bias objective for geomeTRIC. Local validation is still required because ORCA setup depends on the installed executable and calculation environment.