Module: Intrinsic Reaction Coordinate (irc)

The IRC module computes the intrinsic reaction path starting from a transition-state (TS) geometry by following the steepest-descent direction defined by the TS's unique imaginary vibrational mode. This mode corresponds to the negative eigenvalue of the mass-weighted Hessian and provides the correct reaction coordinate. The module currently supports one algorithm: the gs (Gradient Stepping) method.

I. Input Parameters

The IRC task is invoked with:

#irc(method=gs)

The above represents the default settings. Adjustable parameters include:

method

IRC integration algorithm

= gs (default)

II. Output File Contents

Two categories of information are printed:

• Negative Mode Selection

Reports the unique imaginary vibrational mode that defines the reaction coordinate. This mode comes from the negative eigenvalue of the mass-weighted Hessian at the transition state.

Example:

Selected eigenmode #1 (λ = -8.019604e-03 in MW basis)

• IRC Path Iterations

Lists the step-by-step evolution along the IRC path, including: the energy E(Eh), the relative energy change dE(kcal/mol), the maximum force max(|G|), and the root-mean-square force RMS(G). Convergence thresholds for max(|G|) and RMS(G) are also shown; meeting both indicates arrival at a minimum along that direction.

Example:

Iteration E(Eh) dE(kcal/mol) max(|G|) RMS(G)

Convergence thresholds 0.002000 0.000500

0 -382.552193 0.00 0.003218 0.001205

1 -382.557801 -3.52 0.002145 0.000734