Rowan Docs

Intrinsic Reaction Coordinate

Python API · stjames models · API example

How it works

An intrinsic reaction coordinate (IRC) follows the downhill energy path away from a transition state (TS) in both directions. Use it to check which reactant and product structures a proposed TS connects and to inspect how bonds change along the reaction. Rowan returns the TS, structures and energies along both paths, and optional optimized endpoints.

Settings

Start from an optimized transition state, for example from a TS optimization, with one imaginary frequency corresponding to the intended reaction motion.

  • Level of theory: preferably match the TS optimization so the path follows the same energy surface.
  • Re-optimize TS?: refine the input with a TS optimization before tracing the path. Enabled by default in the web form; a starting geometry close to the TS is still needed.
  • Max IRC steps: maximum number of path points in each direction, initially 30 in the web form. Increase it if a path stops before reaching the expected endpoint.
  • Step size (amu1/2·Å): distance between path points in mass-weighted coordinates, initially 0.05 in the web form. Smaller steps improve resolution, especially for proton transfer or sharply curved paths, but may require more steps.
  • Optimize endpoints?: relax the final structure from each direction and calculate its vibrational frequencies. Disabled by default in the web form.

Notes

Rowan checks the TS curvature and requires one significant imaginary mode before tracing the paths. The two directions follow that mode with opposite signs; "forward" and "backward" do not identify reactants and products automatically.

Inspect the structures and energy profile on both sides of the TS. A path can stop at the step limit or after repeated energy increases, so completion alone does not establish that it reached a minimum. Endpoint optimization helps identify the connected species; check their bonding and frequencies rather than assuming they are the intended reactants and products. The path energies describe the potential energy surface, not a free-energy profile or a reaction trajectory over time.

Further reading