Fix load_many crash on single-atom/single-step FCHK optimizations

[Ao-chuba]

This PR fixes a LoadError in load_many for single-atom or single-step FCHK optimizations where the trajectory block is missing.

Changes :
iodata/formats/fchk.py : Added a fallback to construct a single frame trajectory from Current cartesian coordinates, Total Energy, andCartesian Gradientwhen the standard trajectory block is absent but the file is an optimization (FOpt, Scan, IRC).

Tests: Added regression test iodata/test/test_fchk_single_atom.py and data iodata/test/data/atom_opt.fchk to verify the fix and prevent regression.

I ran a manual test script against the newly added regression data (_iodata/test/data/atom_opt.fchk_ ) to verify the fix before submitting. As shown in the screenshot, load_many now correctly parses the file, extracting 1 frame with the expected energy (-0.4665...) and coordinates, instead of crashing.

Closes #386

[tovrstra]

@Ao-chuba Thanks for working on this.

(Minor point: Instead of mentioning the issue being fixed in the PR title, you can add Closes #385 to the initial message. This will link them correctly.)

[Copilot]

Pull request overview

This PR addresses issue #386 by fixing a LoadError in load_many when processing FCHK files from single-atom or single-step optimizations where the standard trajectory block (IRC/Optimization Number of geometries) is missing.

Changes:

• Added fallback logic to construct a single-frame trajectory from current geometry data when trajectory blocks are absent
• Refactored frame creation into a reusable _create_frame helper function
• Added regression test and test data to verify the fix

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
iodata/formats/fchk.py	Implements fallback for missing trajectory blocks and refactors frame creation into _create_frame helper function
iodata/test/test_fchk_single_atom.py	Adds regression test for single-atom optimization loading
iodata/test/data/atom_opt.fchk	Test data file representing a single-atom hydrogen optimization

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[tovrstra]

Copilot picked up a few good points to check.

Just one minor question from the human reviewer. Do you suggest generally splitting up the test files? (So far, we've been keeping tests related to each format together in one file, to reuse utility functions.)

[Ao-chuba]

@tovrstra

Copilot picked up a few good points to check.

Just one minor question from the human reviewer. Do you suggest generally splitting up the test files? (So far, we've been keeping tests related to each format together in one file, to reuse utility functions.)

That makes total sense regarding utility reuse. I completely agree it's better to keep the tests consolidated for consistency.
I have moved the test case directly into iodata/test/test_fchk.py (_and removed test_fchk_single_atom.py) to match the existing structure. I also added the assertions Copilot suggested (verifying zero gradients and ensuring reaction_coordinate is absent)

[tovrstra]

Apologies for having been out of touch for a while. All looks good. Thanks!

[Ao-chuba]

@tovrstra no worries Sir, your Welcome .