MO diagram: on-the-fly fragment orbitals with ECCE-QM for casual diagrams
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
- Issue type
- Feature
- Clarity
- Mostly clear
- Activity status
- Active
- Tech stack
- c
- Domain
- computer-graphics
Research direction
Start by reading the related issues #201 and #252 to understand existing fragment calculations and fragment definitions. Then trace how the MO correlation diagram is built and how ECCE-QM exposes single-point calculations, basis selection, and integrals for orbital overlaps. Done means the diagram can show projected ligand group orbitals using the parent basis and identify the fragment-orbital method it used.
Written by the indexing model from the issue text.
Description
The correlation diagram needs fragment orbitals to show ligand group orbitals properly; today that means fragment calculations made by hand (#201). For a casual diagram, ECCE can make them itself:
- Split the molecule into the metal and its ligands (fragment definitions, #252).
- Run a quick ECCE-QM single point on each ligand at the parent geometry, in the parent calculation's own basis set (ECCE-QM accepts an explicit basis), so fragment and parent orbitals are expanded in the same atomic functions, whatever code ran the parent.
- Project the parent MOs onto the ligand orbitals (overlaps from ECCE-QM's integrals) to get the ligand group orbitals and the correlation lines.
Small ligands (CO, NH3, Cl-, PH3) take seconds each. A Hückel or semi-empirical variant would be faster but uses a different basis, so projections are approximate; ECCE-QM in the parent basis is the default. Quantitative work keeps real fragment calculations (#201). The diagram states which kind of fragment orbitals it shows. Related: #132, #201, #252. Suggested by the maintainer, 2026-10-12.
- Dominant language
- C
- Stars
- 10
- Forks
- 8
- Avg merge
- 21m
- Merged PRs (30d)
- 1
Getting set up
This project ships no dev container, Dockerfile or contributing guide, so setting up is up to you: start from its README, and see our first-contribution guide for the general steps.
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
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