PES Scan

OpenMECP can perform 1D and 2D potential energy surface scans by running a constrained MECP optimization at each point on a grid.

1D Bond Scan

Specify a scan in the *CONSTR section:

*CONSTR
s r atom1 atom2 start num_points step_size
*

Example — scan a C–C bond from 1.0 to 1.95 Å in 20 steps of 0.05 Å:

*CONSTR
s r 1 2 1.0 20 0.05
*

Final geometry at each scan point: scan_1.00.xyz, scan_1.05.xyz, …

*CONSTR
s a atom1 atom2 atom3 start num_points step_size
*

Example — scan a C–C–H angle from 90° to 135° in 10 steps of 5°:

*CONSTR
s a 1 2 3 90 10 5
*

*CONSTR
s d atom1 atom2 atom3 atom4 start num_points step_size
*

Example — full dihedral rotation from 0° to 350° in 36 steps of 10°:

*CONSTR
s d 1 2 3 4 0 36 10
*

Specify two scan lines in *CONSTR:

*CONSTR
s r 1 2 1.0 10 0.1    # First dimension: C–C bond, 10 points
s a 1 2 3 90 5 10     # Second dimension: C–C–H angle, 5 points
*

This performs 10 × 5 = 50 MECP optimizations on a grid. Output geometries are named scan_{val1}_{val2}.xyz.

File	Content
`scan_{value}.xyz` (1D)	MECP geometry at each scan point
`scan_{v1}_{v2}.xyz` (2D)	MECP geometry at each grid point
stdout	Energies and scan coordinate values for each point

The energy values printed to stdout can be used directly to plot the PES scan.

For 2D scans, reduce max_steps per point to keep the total runtime manageable: max_steps = 50.
Use print_level = 0 to suppress per-step output and only log the final energies for each scan point.
Converged MECP geometries from one scan point are used as the starting geometry for the next, which generally speeds up convergence along the scan.