Examples Overview

This section provides complete, ready-to-run examples for locating MECPs with OpenMECP. All examples are also available in the examples/ directory of the repository.

Test System: Phenylcation (C₆H₅⁺)

The primary test case used throughout these examples is the phenylcation, a well-studied 11-atom aromatic cation. The goal is to locate the minimum energy crossing point between the triplet (S = 3) and open-shell singlet (S = 1) spin states.

This system is an excellent benchmark: it is small enough for rapid testing on any hardware, yet exercises all parts of the MECP optimizer, including gradient combination, Hessian updates, and DIIS extrapolation.

Geometry

Save the following as phenylcation.xyz (or embed it directly in the *geom section of your input):

11

C     0.000000000000      1.396613000000      0.000000000000
C     1.209503000000      0.698307000000      0.000000000000
C     1.209503000000     -0.698307000000      0.000000000000
C     0.000000000000     -1.396613000000      0.000000000000
C    -1.209503000000     -0.698307000000      0.000000000000
C    -1.209503000000      0.698307000000      0.000000000000
H     2.150882000000      1.241812000000      0.000000000000
H     2.150882000000     -1.241812000000      0.000000000000
H     0.000000000000     -2.483625000000      0.000000000000
H    -2.150882000000     -1.241812000000      0.000000000000
H    -2.150882000000      1.241812000000      0.000000000000

How to Run

omecp input.inp > output.log

OpenMECP writes a summary line to stdout at each step. Key fields:

Column	Meaning
`Step`	Iteration number
`ΔE`	Energy difference between the two states (target: → 0)
`RMS grad`	RMS effective gradient (target: < `rms_grad` threshold)
`RMS disp`	RMS Cartesian displacement from previous step
`Max disp`	Largest single atomic displacement

Output Files

File	Contents
`{basename}_mecp.xyz`	Final optimized MECP geometry
`running_dir/`	All intermediate QM input/output files
`{basename}_omecp_debug.log`	Step-by-step debug log (if `file_logging = true`)

Available Examples