diff --git a/validation/structure_validation.py b/validation/structure_validation.py
index 3fdd651..2ca2fa5 100644
--- a/validation/structure_validation.py
+++ b/validation/structure_validation.py
@@ -2,22 +2,15 @@
 
 import zipfile
 import tempfile
-from collections import Counter
 import MDAnalysis as mda
 from pathlib import Path
 from typing import Union
-from rdkit import Chem
+from rdkit import Chem, RDLogger
+from rdkit.Chem import AllChem
 
 STRUCTURE_DATASET_SIZE = 78
 
 
-def _heavy_atom_counts_from_smiles(smiles: str) -> Counter | None:
-    mol = Chem.MolFromSmiles(smiles)
-    if mol is None:
-        return None
-    return Counter(atom.GetSymbol() for atom in mol.GetAtoms())
-
-
 def validate_structure_submission(
     structure_predictions_file: Union[str, Path],
     expected_ids: set[str] | None = None,
@@ -78,27 +71,37 @@ def validate_structure_submission(
                             if len(u.segments) > 2:
                                 errors.append(f"{name}: Found {len(u.segments)} chains, expected 2 or fewer")
 
-                            # 4. Check ligand matches expected SMILES by heavy-atom composition
+                            # 4. Check ligand graph matches expected SMILES connectivity
                             if expected_ligand_smiles is not None:
                                 pdb_id = Path(name).stem
                                 expected_smi = expected_ligand_smiles.get(pdb_id)
                                 if expected_smi is not None:
-                                    expected_counts = _heavy_atom_counts_from_smiles(expected_smi)
-                                    if expected_counts is None:
+                                    ref_mol = Chem.MolFromSmiles(expected_smi)
+                                    if ref_mol is None:
                                         errors.append(
                                             f"{name}: Could not parse expected SMILES for '{pdb_id}'"
                                         )
                                     else:
-                                        obs_counts = Counter(
-                                            elem for elem in ligands.elements
-                                            if elem.strip() not in ("H", "h", "")
+                                        lig_pdb_path = Path(tmpdir) / f"{pdb_id}_lig.pdb"
+                                        ligands.write(str(lig_pdb_path))
+                                        lig_mol = Chem.MolFromPDBFile(
+                                            str(lig_pdb_path), removeHs=True, sanitize=False
                                         )
-                                        if obs_counts != expected_counts:
+                                        if lig_mol is None:
                                             errors.append(
-                                                f"{name}: Ligand heavy-atom composition mismatch. "
-                                                f"Expected {dict(sorted(expected_counts.items()))}, "
-                                                f"got {dict(sorted(obs_counts.items()))}"
+                                                f"{name}: RDKit could not parse LIG residue"
                                             )
+                                        else:
+                                            try:
+                                                RDLogger.DisableLog("rdApp.*")
+                                                AllChem.AssignBondOrdersFromTemplate(ref_mol, lig_mol)
+                                                RDLogger.EnableLog("rdApp.*")
+                                            except ValueError:
+                                                RDLogger.EnableLog("rdApp.*")
+                                                errors.append(
+                                                    f"{name}: Ligand connectivity does not match "
+                                                    f"expected SMILES '{expected_smi}'"
+                                                )
 
                         except Exception as e:
                             errors.append(f"{name}: MDAnalysis failed to parse file: {e}")