IRC Path Following

The IRC true directive triggers bidirectional IRC from the provided geometry. The starting geometry should be a pre-converged TS.

IRC Path Following from a Transition State

task: RHF STO-3G

GEOMETRY
  O   0.000000   0.000000   0.000000
  H   0.800000   0.000000   0.400000
  H  -0.800000   0.000000   0.400000
END

OPT
  IRC     true
  IRCDir  +1
END

Notes

  • The IRC true directive triggers bidirectional IRC from the provided geometry.
  • Restrict to one direction with IRCDir +1 (forward) or IRCDir -1 (reverse).
  • The starting geometry should be a pre-converged TS (imaginary frequency expected).
  • Uses Gonzalez–Schlegel (1990) mass-weighted IRC with predictor-corrector steps.
  • Output includes per-step energies, geometries, and a formatted path summary.

Sella TS + Frequency Verification + IRC Workflow

Step 1 — TS Optimisation with Sella + DLC:

task: TS SELLA RHF STO-3G

GEOMETRY
  H  -1.200000   0.000000   0.000000
  H   0.000000   0.000000   0.000000
  H   1.200000   0.000000   0.000000
END

OPT
  Coord    dlc
  MaxCycle 150
END

Step 2 — Frequency Analysis on Converged TS Geometry:

task: FREQ RHF STO-3G

GEOMETRY
  # Paste the optimized TS geometry from step 1
  H  ...
  H  ...
  H  ...
END

Step 3 (optional) — IRC from the TS:

task: RHF STO-3G

GEOMETRY
  # Same TS geometry
  H  ...
  H  ...
  H  ...
END

OPT
  IRC true
END

Notes

  • IRC requires a pre-converged TS geometry
  • The corrector uses up to 10 iterations per IRC step with perpendicular gradient tolerance of 1e-6
  • Bidirectional by default; use IRCDir +1 or -1 to restrict
  • Output includes per-step energies, geometries, and formatted path summary