ORCA NEB-TS Transition State Skill

When to Use

Use this skill when the user wants to:

• Find a transition state between two structures
• Calculate a reaction barrier
• Map a minimum energy path (MEP) between reactant and product

Requirements: the user MUST provide both a reactant and a product structure. If only one structure is available, ask for the other before proceeding.

How NEB-TS Works

1. ORCA interpolates images between reactant and product geometries
2. NEB optimization finds the minimum energy path
3. Climbing-image NEB (CI-NEB) refines the highest-energy image
4. The TS is characterized by exactly one imaginary frequency

MCP Tool Examples — proven Expanse submission flow

Use catgo_workflow (graph-based), NOT catgo_workflow_engine (task-based). NEB-TS needs TWO structure inputs (reactant + product) wired to separate input ports. The graph-based tool lets you build that wiring explicitly. Param keys also differ: graph-based uses method/basis, task-based uses orca_method/orca_basis.

Step 1: Load both structures and capture their JSON

NEB-TS needs reactant and product. The create action auto-captures only the viewer structure, so capture each one separately and pass them inline.

Load reactant into viewer (file or PubChem):

catgo_structure(action: "load_file", file_content: "<reactant xyz>", file_format: "xyz")

Capture its JSON:

catgo_view(action: "get_state")

Save the resulting structure JSON as <reactant_json>.

Then load and capture the product the same way → <product_json>.

Step 2: Find Expanse session_id

curl -s http://localhost:8000/api/hpc/connections

Copy the session_id for host: login.expanse.sdsc.edu.

Step 3: Create the workflow

catgo_workflow(action: "create", name: "NEB-TS Cl- + CH3Br")

Note the auto-created structure_input node ID. You'll either reuse it for the reactant (and add a second structure_input for the product) or remove it and add two fresh ones.

Step 4: Wire reactant + product → orca_neb_ts

catgo_workflow(action: "batch", workflow_id: "<wf_id>", operations: [
  {"op": "add_node", "node_type": "structure_input", "label": "reactant",
   "params": {"structure_json": "<reactant_json>"}},
  {"op": "add_node", "node_type": "structure_input", "label": "product",
   "params": {"structure_json": "<product_json>"}},
  {"op": "add_node", "node_type": "orca_neb_ts", "label": "neb",
   "params": {
     "software": "orca",
     "method": "B3LYP",
     "basis": "def2-SVP",
     "dispersion": "D4",
     "charge": -1,
     "multiplicity": 1,
     "nimages": 8,
     "ts_opt": true,
     "neb_cycles": 100
   }},
  {"op": "connect", "from_id": "reactant", "to_id": "neb",
   "from_handle": "structure", "to_handle": "structure"},
  {"op": "connect", "from_id": "product", "to_id": "neb",
   "from_handle": "structure", "to_handle": "structure_product"}
])

The two connect ops are critical — to_handle must be structure for the reactant edge and structure_product for the product edge. (Verify with node_details(orca_neb_ts) — input handles are ["structure", "structure_product"].) If both go to structure the neb task sees a list and uses only the first.

Step 5: Optional — chain a freq node to verify the TS

{"op": "add_node", "node_type": "freq", "label": "freq_ts",
 "params": {"software": "orca", "method": "B3LYP", "basis": "def2-SVP", "charge": -1, "multiplicity": 1}},
{"op": "connect", "from_id": "neb", "to_id": "freq_ts",
 "from_handle": "structure", "to_handle": "structure"}

A valid TS shows exactly one imaginary frequency.

Step 6: Run with the full HPC run_config

NEB-TS is much more expensive than a single opt — bump walltime accordingly, and consider partition: "shared" or "compute" instead of "debug" (which caps at 30 min). Read server/templates/orca_generic.sh and pass its contents as default_template.

catgo_workflow(action: "run", workflow_id: "<wf_id>", run_config: {
  "execution_mode": "hpc",
  "default_session_id": "<expanse_session_id>",
  "base_work_dir": "/expanse/lustre/projects/sdp126/jyang25/ORCA/catgo",
  "default_job_params": {
    "nodes": 1, "ntasks": 8, "cpus_per_task": 1,
    "walltime": "08:00:00", "partition": "shared"
  },
  "cluster_configs": {
    "<expanse_session_id>": {
      "account": "sdp126",
      "partition": "shared",
      "module_loads": "module load cpu/0.17.3b\nmodule load gcc/10.2.0/npcyll4\nexport PATH=$HOME/openmpi-4.1.8/bin:$PATH\nexport LD_LIBRARY_PATH=$HOME/openmpi-4.1.8/lib:$LD_LIBRARY_PATH",
      "orca_dir": "/home/jyang25/orca_6_1_1_RRP8",
      "default_template": "<contents of server/templates/orca_generic.sh>",
      "default_job_params": {
        "nodes": 1, "ntasks": 8, "cpus_per_task": 1,
        "walltime": "08:00:00", "partition": "shared"
      }
    }
  }
})

The local-scratch template stages I/O to $TMPDIR/orca_$SLURM_JOB_ID and copies results back. Required on Expanse — Lustre kills ORCA's many-small-file I/O during the per-image SCFs.

Step 7: Monitor and pull results

catgo_workflow(action: "status", workflow_id: "<wf_id>")

mkdir -p ./local_run
for f in ORCA.out ORCA_MEP_trj.xyz ORCA.NEB.log \
         ORCA.property.json ORCA.json; do
  curl -s -X POST http://localhost:8000/api/hpc/files/read-content \
    -H 'Content-Type: application/json' \
    -d "{\"session_id\":\"<expanse_session_id>\",\"file_path\":\"<work_dir>/$f\"}" \
    > ./local_run/$f
done

Step 8: Parse with OPI

OPI gives a per-image energy curve and the converged-TS imaginary-frequency check without regex. Requires pip install orca-pi.

import sys
sys.path.insert(0, ".claude/skills")  # for the _shared helper
from _shared.orca_opi import parse_local

out = parse_local("./local_run")

# Per-image energy curve — geometries[i] holds each image's properties
energies_eh = [
    g.single_point_data.finalenergy
    for g in out.results_properties.geometries
]
print("MEP energies (Eh):", energies_eh)
print("Barrier (Eh):", max(energies_eh) - energies_eh[0])

# Converged TS imaginary check (after NEB-TS refinement step)
ts_freqs = out.results_properties.geometries[-1].thermochemistry_energies[0].freq
n_imag = sum(1 for f in ts_freqs if f < 0)
assert n_imag == 1, f"Expected 1 imaginary mode, got {n_imag}"
print("Imaginary mode (cm^-1):", min(ts_freqs))

NEB does not always produce a thermochemistry block (depends on whether NEB-TS finished its frequency confirmation step). If thermochemistry_energies is missing, run a follow-up freq node on the converged TS structure.

Viewing the MEP curve in the IDE

from _shared.orca_opi import quick_plot_neb_mep, show_png
png = quick_plot_neb_mep(out)            # writes ./local_run/neb_mep.png
show_png(png, "NEB-TS MEP")              # prints `![NEB-TS MEP](local_run/neb_mep.png)`

After running this, reply to the user with the markdown link the script printed so Claude Code renders the figure inline in chat.

Submission gotchas (real failures we hit)

• catgo_workflow_engine.add_task doesn't auto-attach structures → "No input structure provided".
• Connecting both reactant and product to to_handle: "structure" (default) → neb task sees a list, uses only the first → garbage path. Use reactant and product handles explicitly.
• partition=workq (Shaheen default) is invalid on Expanse → use debug/shared/compute.
• partition=debug capped at 30 min — use shared for any real NEB-TS run.
• Missing account=sdp126 → "Invalid account or account/partition combination".
• Missing module_loads + orca_dir → orca not on PATH; per-image SCFs silently produce nothing.
• After re-connecting to Expanse, the session_id changes — re-discover via /api/hpc/connections and update default_session_id + cluster_configs key.

Canonical NEB-TS node params (what the engine actually reads)

Parameter	Default	Description
method	r2SCAN-3c	DFT functional
basis	def2-SVP	Basis set (omit for composite methods)
charge / multiplicity	0 / 1
dispersion	(none)	D4 | D3BJ | D3 — use this field, NOT extra_keywords
grid	DefGrid2	DefGrid1/2/3
nimages	8	Number of interpolated images
ts_opt	true	Switch to CI-NEB after convergence
neb_cycles	100	NEB iteration cap
interpolation	"IDPP"	Initial-path interpolation method
num_cores / max_core_mb	8 / 4000

⚠️ extra_keywords, extra_blocks, neb_images, neb_convergence are NOT read by the engine — they're phantom params from earlier skill versions. Use the names above. There is no neb_convergence knob currently exposed.

About OPI input builders

OPI (pip install orca-pi) ships a typed BlockNeb builder. However, the catgo backend's orca_neb_ts node already emits its own %neb block from node params (neb_images, neb_convergence, etc.). Pasting an OPI-built %neb block via extra_blocks would produce two %neb blocks in the same .inp, which is undefined behavior.

For this skill, stick with node params for %neb content and use extra_blocks only for %output. The OPI parsing wins (per-image energy curve, TS imaginary-mode check) still apply. If you need a knob BlockNeb exposes that the node params don't (interpolation, springconst, ts_inputhess, zoom_*, etc.), open that as a node-def gap rather than dual-emitting blocks.

Image count guidelines

System size	Recommended images
Small molecule (<15 atoms)	6-8
Medium molecule (15-50 atoms)	8-12
Large molecule (>50 atoms)	12-16

More images = smoother path but higher cost (each image is a full DFT calc).

Common Reaction Types

SN2 reaction

• Charge: -1 (incoming nucleophile)
• Check that leaving group bond elongates along path

Bond dissociation / formation

• Usually neutral, singlet
• Consider if radical pathway needs multiplicity: 3 (triplet)

Proton transfer

• Include dispersion: dispersion: "D3BJ" (or "D4" for newer Grimme correction)
• Solvent: CPCM is currently a node-def gap on neb_ts (no first-class field, and extra_keywords is unread). Workaround: gas-phase NEB-TS, then refine TS energy with a CPCM single-point.

Troubleshooting

NEB does not converge

• Increase neb_images (more interpolation points)
• Use a better starting path (optimize reactant and product first)
• Try neb_convergence: "loose" for initial run, then tighten

Wrong TS found

• Check the imaginary frequency mode -- does it correspond to the expected bond breaking/forming?
• Try different initial interpolation (reorder atoms so they correspond)

Too expensive

• Screen with HF-3c or orca_method: "PBE", orca_basis: "def2-SVP" first
• Refine with better method only on the TS geometry (single-point)

Important Notes

• Reactant and product MUST have the same atoms in the same order
• Both structures should be pre-optimized at the same level of theory
• ORCA NEB-TS automatically switches to CI-NEB after initial convergence
• The barrier height is the energy difference between the TS and the reactant