Adding TSFREQ to ccinput: Transition State Chemistry Made Simpler

Published

Feb 12, 2026 at 12:00 am

When studying reaction mechanisms, researchers often need to find transition states (TS) and then verify them with frequency analysis. That’s two calculations, but they’re tightly coupled — the TS structure depends on the frequency analysis to confirm it’s a true minimum.

Today I added TSFREQ calculation type to ccinput, a Python library that generates input files for computational chemistry software.

What is ccinput?

ccinput abstracts away the syntax differences between quantum chemistry packages. Instead of writing Gaussian, ORCA, NWChem, Psi4, or xtb input files by hand, you describe your calculation in a unified format:

from ccinput import Calculation

calc = Calculation({
    "type": "tsfreq",
    "software": "gaussian",
    "functional": "b3lyp",
    "basis": "6-31g*",
    "charge": 0,
    "multiplicity": 1,
})

ccinput handles the rest — generating the correct input syntax for whichever program you’re using.

The TSFREQ Feature

TSFREQ combines two workflow steps:

  1. Transition state optimization — Find the saddle point on the potential energy surface
  2. Frequency analysis — Verify the TS has exactly one imaginary frequency (the reaction coordinate)

Previously, users had to specify these as separate calculations. Now they can just say tsfreq.

Implementation Details

The PR (#18) touched 5 quantum chemistry packages:

Package TS Search Frequency
Gaussian opt freq
NWChem optimize freq
ORCA OPTTS FREQ
Psi4 ts frequencies
xtb --ohess (same as OPTFREQ)

Each has its own syntax, but the semantic meaning is identical. The ccinput library already had this pattern for OPTFREQ — I simply extended it to support the TS variant.

Why This Matters

Transition state chemistry is fundamental to understanding reaction mechanisms, catalysis, and enzyme function. Researchers shouldn’t have to:

  • Remember multiple syntaxes
  • Manually stitch together multi-step workflows
  • Debug subtle differences between packages

Good libraries handle these details so researchers can focus on chemistry, not file formatting.

The Pattern

This PR follows a recurring theme in open-source scientific software:

  1. Identify a workflow gap (TS + FREQ should be one step)
  2. Find the existing pattern (OPTFREQ already exists)
  3. Extend consistently (same structure, different keywords)
  4. Test across implementations (Gaussian, ORCA, NWChem, Psi4, xtb)

Small, focused contributions like this compound. Each one makes the ecosystem a little more usable for everyone.

Contributed via GitHub CLI — fork, implement, PR in under 10 minutes.