feat: implement fuzzy bond order analysis

[newtontech]

Summary

This PR implements fuzzy bond order analysis based on electron density, providing a robust measure of bond strength using fuzzy atom partitioning. The implementation includes the FuzzyAtom dataclass, fuzzy bond order calculation functions, and integrates the Bonding class into the main module API.

Changes

• pymultiwfn/init.py: Export Bonding class for API compatibility (from pymultiwfn import Bonding)
• pymultiwfn/bonding/fuzzy.py: Complete fuzzy bond order implementation
  • FuzzyAtom dataclass with fuzzy partition factor and van der Waals radii
  • fuzzy_bond_order() function using density and overlap matrices
  • calculate_fuzzy_bond_order_matrix() for computing bond orders between all atom pairs
• pymultiwfn/bonding/bonding.py: Added get_fuzzy_bond_order() and get_fuzzy_bond_order_matrix() methods
• tests/bonding/test_fuzzy_bond_order.py: Comprehensive test suite with 16 tests

Testing

All 16 tests pass, covering:

• Fuzzy atom definition and boundaries
• Overlap population calculations
• Bond order calculations for single (H₂), double (C₂H₄), triple (N₂), and aromatic (benzene) bonds
• Bond order matrix generation (shape, symmetry, diagonal)
• Input validation (negative indices, out-of-range, same atom)

16 passed in 0.06s

API Usage

from pymultiwfn import Bonding
bond = Bonding("molecule.fch")
fbo = bond.get_fuzzy_bond_order(atom_i=0, atom_j=1)
matrix = bond.get_fuzzy_bond_order_matrix()

Fixes #2

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