Reaction Path & NEB

Coordinate Driving

Coordinate driving systematically varies an internal coordinate (bond, angle, or dihedral) in a series of constrained MECP optimizations. This traces a reaction path on the crossing seam.

Keywords

drive_type       = bond        # bond | angle | dihedral
drive_atoms      = 1,2         # comma-separated atom indices
drive_start      = 1.0         # starting value (Å or degrees)
drive_end        = 2.5         # ending value
drive_steps      = 20          # number of steps

Example — C–N Bond Elongation

drive_type   = bond
drive_atoms  = 1,2
drive_start  = 1.4
drive_end    = 2.4
drive_steps  = 20

At each of the 20 steps, OpenMECP runs a constrained MECP optimization with the C–N bond fixed at the current driving value. The converged geometry is then used as the starting point for the next step.

Output: Energy profile along the driven coordinate printed to stdout; geometries saved to running_dir/.

Nudged Elastic Band (NEB)

NEB optimizes an entire reaction path simultaneously by connecting a chain of geometries ("images") with spring forces that keep them evenly spaced along the path.

Setup

drive_type   = bond
drive_atoms  = 1,2
drive_start  = 1.0
drive_end    = 2.0
drive_steps  = 10    # number of images on the path

When the run_mode is set to path_optimization, OpenMECP:

1. Creates an initial path via coordinate driving.
2. Applies simplified NEB spring forces between adjacent images.
3. Optimizes each image's position on the crossing seam.

Current Limitations: The NEB implementation uses a simplified spring model. A full NEB with perpendicular energy gradient projection is planned for a future release.

Output: Smooth energy profile along the optimized path; geometry files for each image.

Choosing Between Methods