⏳ MECI Optimization¶

📖 In the MECI Optimization step, each cluster’s centroid from the previous stage is refined to locate a Minimum Energy Conical Intersection (MECI) using multireference electronic structure methods such as CASSCF, FOMO-CASCI, or hhTDA.

Concept Overview¶

Each cluster’s centroid geometry serves as an initial guess for MECI optimization.
MECI searches refine the geometry to the point of degeneracy between two electronic states with identical multiplicity.
The resulting optimized MECIs represent key regions of the intersection seam and are used as inputs for cone sampling.
These structures help map the accessible parts of the conical intersection space.

Step-By-Step MECI Optimization¶

Create directories for each centroid
```
mkdir ciopt0 ciopt1 ciopt2 ...
```

Move each centroid into its directory

mv centroid0.xyz ciopt0/
mv centroid1.xyz ciopt1/

Create the TeraChem input file tc.in

basis 6-31gs
charge 0
spinmult 1
method rhf
precision mixed
scf diis+a
closed 20
active 5
casscf yes
spinmult 1
cassinglets 4
castarget 0
castargetmult 1
precision mixed
coordinates centroid0.xyz
run ciopt
nacstate1 2
nacstate2 3
casguess c0.casscf
casscftrustmaxiter 1000
casscfmaxiter 1000
casscforbnriter 1000
cpsacasscfmaxiter 1000
cphfiter 1000
end

Add orbital guess files if applicable

<molecule>/seam_sampling/clustering/ciopt0/c0
<molecule>/seam_sampling/clustering/ciopt0/c0.casscf

Create submit.sh for each MECI optimization

#!/bin/bash
#SBATCH -p l40-gpu
#SBATCH -N 1
#SBATCH -n 4
#SBATCH -J centroid0
#SBATCH --mem=50G
#SBATCH -t 30:00:00
#SBATCH --qos gpu_access
#SBATCH --gres=gpu:1

module load tc/25.03
terachem tc.in > tc.out

Submit jobs for all centroids
```
sbatch submit.sh
```

Outputs¶

scr.centroid#/optim.xyz → optimized MECI geometry
tc.out → full TeraChem output log
These MECIs are used as input structures for Cone Sampling.

Continue to 🍦 Cone Sampling →