IRC Method: GS

The GS (Gradient Stepping) method is the default integrator for intrinsic reaction coordinate (IRC) calculations. It follows the transition state geometry downhill along the unique imaginary vibrational mode and refines the trajectory using perpendicular force correction and adaptive step-size control. This method is suitable for verifying whether the transition state correctly connects reactants and products, and for analyzing the reaction pathway's structural and energetic evolution.

I. Default Parameters

#irc(method=gs,
    target_mode=1,
    init_delta_E_mEh=2.0,
    use_quadratic_init=true,
    sd_len_bohr=0.150,
    sd_len_min_bohr=0.050,
    sd_len_max_bohr=0.300,
    corr_scale=0.333,
    grow=1.20,
    shrink=0.80,
    do_parabolic_fit=true,
    tol_maxf=0.002,
    tol_rmsf=0.000500,
    max_points=100,
    print_each=true,
    write_traj=true)

II. Adjustable Parameters

target_mode

Selects the imaginary vibrational mode used to initiate the IRC (1 = most negative eigenvalue).

init_delta_E_mEh

Energy target for determining the magnitude of the initial displacement along the imaginary mode.

use_quadratic_init

Enables curvature-based initialization using Hessian information

• = true (default)
• = false

sd_len_bohr

Base step length controlling the primary stepping distance along the reaction coordinate.

sd_len_min_bohr

Minimum allowed adaptive step size to prevent stagnation during integration

sd_len_max_bohr

Maximum allowed adaptive step size to avoid unphysical structural jumps.

corr_scale

Relative scaling factor for the perpendicular correction applied to remove drift from the true reaction path.

grow

Adaptive scaling factor that increases the step length when the energy decreases smoothly.

shrink

Adaptive scaling factor that reduces the step length when uphill movement or unstable stepping is detected.

do_parabolic_fit

Enables parabolic interpolation when an uphill step occurs to recover the downhill direction

• = true (default)
• = false

tol_maxf

Convergence threshold for the maximum force component along the path.

tol_rmsf

Convergence threshold for the RMS force component; must be satisfied together with tol_maxf.

max_points

Maximum number of macro iterations for each direction (forward and backward).

print_each

Controls whether iterative energy and gradient information is printed during integration

• = true (default)
• = false

write_traj

Controls writing the IRC trajectory to disk (forward, backward, and merged path)

• = true (default)
• = false