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Intrinsic Reaction Coordinate (IRC)

Overview

The Intrinsic Reaction Coordinate (IRC) module follows the minimum energy path (MEP) from a transition state (TS) geometry downhill toward both the reactant and product minima. The IRC path is defined as the mass-weighted steepest descent path originating from the TS saddle point, and it provides definitive verification that a given transition state connects the expected reactant and product structures.

Starting from the TS, the IRC integrator displaces the geometry along the unique imaginary vibrational mode (the eigenvector corresponding to the single negative eigenvalue of the mass-weighted Hessian) and then follows the gradient downhill in both the forward and backward directions until local minima are reached or the maximum number of path points is exceeded.

Available Methods

MAPLE supports four IRC integration algorithms:

Method Full Name Description
GS Gonzalez–Schlegel Default method. Mass-weighted coordinates with pivot-step and constrained optimization on a hypersphere. Robust and well-tested.
LQA Local Quadratic Approximation Fits a local quadratic surface to the PES at each point. Can be faster than GS on smooth surfaces.
HPC Hessian Predictor–Corrector Uses Hessian information for both predictor and corrector steps. Good balance of speed and accuracy.
EulerPC Euler Predictor–Corrector Simplest predictor-corrector approach using Euler integration. Most robust for difficult or highly anharmonic surfaces.

General Usage

The IRC task is invoked with the #irc keyword, specifying the desired method:

#model=ani1xnr
#irc(method=gs)
#device=gpu0

XYZ /path/to/transition_state.xyz

The input geometry must be a valid transition-state structure with exactly one imaginary vibrational frequency. The IRC module reads this geometry, identifies the imaginary mode from the Hessian, and integrates the reaction path in both directions.

Important

The input structure must be a converged transition state with exactly one imaginary frequency. Running IRC on a geometry that is not a true first-order saddle point will produce unreliable results. Always verify the TS with a frequency calculation before running IRC.

Output Files

The IRC module produces the following output files:

  • *_forward.xyz — Multi-frame XYZ trajectory of the forward IRC path (from TS toward one minimum).
  • *_backward.xyz — Multi-frame XYZ trajectory of the backward IRC path (from TS toward the other minimum).
  • *_full.xyz — Combined trajectory containing both forward and backward paths, with the TS at the center.
  • *.out — Main text output with step-by-step energy, gradient, and convergence information.

Workflow

The recommended workflow for verifying a transition state using IRC is:

  1. TS Search — Locate the transition state using one of the TS methods (P-RFO, NEB, Dimer, String/GSM).
  2. Frequency Check — Run a frequency calculation on the optimized TS. Confirm that there is exactly one imaginary frequency and that its vibrational mode corresponds to the expected reaction coordinate.
  3. IRC Calculation — Run IRC from the verified TS geometry.
  4. Verify Endpoints — Inspect the forward and backward IRC endpoints to confirm they correspond to the expected reactant and product structures.