Gaussian Frontier Orbitals

Gaussian HOMO/LUMO analysis

Use this skill when .fch files need HOMO/LUMO indices, orbital cubes, energy data, or rendered frontier-orbital images.

Safety gate: wavefunction type

Automatic index selection is valid only for a restricted closed-shell wavefunction: HOMO = total electrons / 2 and LUMO = HOMO + 1. Before using it, inspect the FCH occupation/electron information.

• For UHF, ROHF, open-shell, spin-polarized, or fractional occupations, stop automatic selection.
• Ask the user for the required alpha/beta spin channel and orbital indices, or use an occupation-aware parser.
• Never silently apply the closed-shell formula to UHF, ROHF, or fractional systems.

Workflow

Run the bundled closed-shell exporter from the analysis folder:

python /scripts/run_orbitals.py --input fch --output output

It carries its own export logic and does not require this repository's root scripts.

1. Locate Multiwfn through MULTIWFN_BIN, then PATH, then a user-provided path. For optional graphics, resolve VMD_BIN similarly; PNG conversion requires Pillow.
2. For every fch/.fch, create an isolated run directory and record the selected HOMO/LUMO indices in a root-level orbital_indices.txt and a per-molecule log.
3. Export exactly the selected HOMO and LUMO orbital cube files through Multiwfn. Use deterministic names such as _orb######.cub and do not overwrite an existing final cube without permission.
4. Extract the selected orbital energies and write _energies.txt, including orbital indices, units, and spin channel.
5. If graphics are requested, render the two cubes to _HOMO.tga and _LUMO.tga, convert them to corresponding PNG files, verify readability, and remove only the temporary TGA files. The two orbital phases use red #CE3B58 for positive and blue #4A65A0 for negative.

Output contract

For a closed-shell molecule, require 2 orbital cubes, orbital_indices.txt, and _energies.txt; graphics add 2 PNG files. Stop on missing cubes, inconsistent indices, VMD failures, or Multiwfn errors. Some Multiwfn 3.8 Windows builds return an EOF code after writing both orbital cubes; accept that return only when both newly generated cubes and the export log are validated.

Reporting

Keep long Multiwfn output in molecule logs. Finish with a short table of selected orbitals and generated files; refer ESP requests to $gaussian-esp-vmd and ELF requests to $gaussian-elf.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Nanhuai-X
• Source: Nanhuai-X/gaussian-molecule-workflow
• License: MIT

Install and usage instructions live in the source repository linked above.