fix zero division error in conformers

[shuyana]

Changelogs

• Fixed a division-by-zero error in dm.conformers.generate() when minimize_energy is set to True and the molecule has no rotatable bonds. Below is a minimal example that reproduces the error:

  import datamol as dm
  
  mol = dm.to_mol("c1ccccc1")
  dm.conformers.generate(mol, minimize_energy=True)

  Error message:

  /workspaces/datamol/datamol/conformers/_conformers.py:214: RuntimeWarning: invalid value encountered in scalar divide
  if E - minE <= ewindow and (E - minE) / rotatable_bonds <= eratio
  /workspaces/datamol/datamol/conformers/_conformers.py:214: RuntimeWarning: divide by zero encountered in scalar divide
    if E - minE <= ewindow and (E - minE) / rotatable_bonds <= eratio
  

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Checklist:

• Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• Update the API documentation is a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs below.
• If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

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discussion related to that PR

[hadim]

Thanks, @shuyana and nice catch for the bug!

Do you mind adding a test for it?

[codecov]

Codecov Report

Merging #209 (100fd4c) into main (2619c61) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #209   +/-   ##
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  Coverage   91.94%   91.94%           
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  Files          46       46           
  Lines        3826     3826           
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  Misses        308      308           

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unittests	91.94% <ø> (ø)

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Files Changed	Coverage Δ
datamol/conformers/_conformers.py	93.16% <ø> (ø)

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[shuyana]

Thank you for the quick reply!
I think the existing test, test_conformer_energy(), covers the case where rotatable_bonds is not zero.
So, I've added a test for the case where rotatable_bonds is zero:

def test_conformer_no_rotatable_bonds():
    mol = dm.to_mol("c1ccccc1")

    random.seed(42)
    np.random.seed(42)
    mol1 = dm.conformers.generate(mol, minimize_energy=True)

    random.seed(42)
    np.random.seed(42)
    mol2 = dm.conformers.generate(mol, minimize_energy=True, eratio=3)

    # `eratio` should be ignored for this molecule as it has no rotatable bonds
    assert mol1.GetNumConformers() == mol2.GetNumConformers()

This test fails without the fix because (E - minE) / rotatable_bonds is inf and always greater than the finite eratio for mol2.
On the other hand, the test passes with the fix because (E - minE) / rotatable_bonds is not used when rotatable_bonds is zero.

If you have any comments or suggestions, please let me know!

[hadim]

Thanks. That sounds good to me.

I am good to merge here once the ruff and black CI errors are gone (likely as simple as running those locally).

[shuyana]

Thanks for the review!
I've fixed the CI errors and pushed the changes.