Repository: aspuru-guzik-group/selfies

Issue:
Kekulization Failure
When trying to run this line of code:

`sf.encoder('Cc1ccc(NC(=O)c2ccc(-c3[c]n(Br)ccs[nH]3)c(C(F)(F)F)c2)cc1Nc1nccc(-c2cccnc2)n1')`

I get this error even though RDKit smiles parser is able to produce a Mol object from the same smiles string

EncoderError: kekulization failed
	SMILES: Cc1ccc(NC(=O)c2ccc(-c3[c]n(Br)ccs[nH]3)c(C(F)(F)F)c2)cc1Nc1nccc(-c2cccnc2)n1

Discussion:

Patch:
diff --git a/selfies/constants.py b/selfies/constants.py
index 597ff5e..0fcd597 100644
--- a/selfies/constants.py
+++ b/selfies/constants.py
@@ -21,6 +21,13 @@ AROMATIC_VALENCES = {
     "O": (2, 4), "S": (2, 4), "Se": (2, 4), "Te": (2, 4)
 }
 
+VALENCE_ELECTRONS = {
+    "B": 3, "Al": 3,
+    "C": 4, "Si": 4,
+    "N": 5, "P": 5, "As": 5,
+    "O": 6, "S": 6, "Se": 6, "Te": 6
+}
+
 AROMATIC_SUBSET = set(e.lower() for e in AROMATIC_VALENCES)
 
 # =============================================================================
diff --git a/selfies/mol_graph.py b/selfies/mol_graph.py
index b491333..96bc7c2 100644
--- a/selfies/mol_graph.py
+++ b/selfies/mol_graph.py
@@ -4,7 +4,7 @@ from typing import List, Optional, Union
 from dataclasses import dataclass, field
 
 from selfies.bond_constraints import get_bonding_capacity
-from selfies.constants import AROMATIC_VALENCES
+from selfies.constants import AROMATIC_VALENCES, VALENCE_ELECTRONS
 from selfies.utils.matching_utils import find_perfect_matching
 
 
@@ -254,7 +254,7 @@ class MolecularGraph:
 
         ds = self._delocal_subgraph
         kept_nodes = set(itertools.filterfalse(self._prune_from_ds, ds))
-
+        
         # relabel kept DS nodes to be 0, 1, 2, ...
         label_to_node = list(sorted(kept_nodes))
         node_to_label = {v: i for i, v in enumerate(label_to_node)}
@@ -265,7 +265,7 @@ class MolecularGraph:
             label = node_to_label[node]
             for adj in filter(lambda v: v in kept_nodes, ds[node]):
                 pruned_ds[label].append(node_to_label[adj])
-
+        
         matching = find_perfect_matching(pruned_ds)
         if matching is None:
             return False
@@ -288,18 +288,33 @@ class MolecularGraph:
         adj_nodes = self._delocal_subgraph[node]
         if not adj_nodes:
             return True  # aromatic atom with no aromatic bonds
-
+        
         atom = self._atoms[node]
         valences = AROMATIC_VALENCES[atom.element]
-
+        
         # each bond in DS has order 1.5 - we treat them as single bonds
         used_electrons = int(self._bond_counts[node] - 0.5 * len(adj_nodes))
-
+        
         if atom.h_count is None:  # account for implicit Hs
             assert atom.charge == 0
             return any(used_electrons == v for v in valences)
         else:
             valence = valences[-1] - atom.charge
             used_electrons += atom.h_count
-            free_electrons = valence - used_electrons
-            return not ((free_electrons >= 0) and (free_electrons % 2 != 0))
+            
+            # count the total number of bound electrons of each atom
+            bound_electrons = (max(0, atom.charge) + atom.h_count 
+                               + int(self._bond_counts[node]) 
+                               + int(2 * (self._bond_counts[node] % 1)))
+            
+            # calculate the number of unpaired electrons of each atom
+            radical_electrons = (max(0, VALENCE_ELECTRONS[atom.element] 
+                                 - bound_electrons) % 2)
+            
+            # unpaired electrons do not contribute to the aromatic system
+            free_electrons = valence - used_electrons - radical_electrons
+            
+            if any(used_electrons == v - atom.charge for v in valences):
+                return True
+            else:
+                return not ((free_electrons >= 0) and (free_electrons % 2 != 0))
\ No newline at end of file


Test patch:
diff --git a/tests/test_specific_cases.py b/tests/test_specific_cases.py
index 374e439..22ee652 100644
--- a/tests/test_specific_cases.py
+++ b/tests/test_specific_cases.py
@@ -8,6 +8,12 @@ def decode_eq(selfies, smiles):
     return s == smiles
 
 
+def roundtrip_eq(smiles_in, smiles_out):
+    sel = sf.encoder(smiles_in)
+    smi = sf.decoder(sel)
+    return smi == smiles_out
+
+
 def test_branch_and_ring_at_state_X0():
     """Tests SELFIES with branches and rings at state X0 (i.e. at the
     very beginning of a SELFIES). These symbols should be skipped.
@@ -330,6 +336,7 @@ def test_old_symbols():
     except Exception:
         assert False
 
+
 def test_large_selfies_decoding():
     """Test that we can decode extremely large SELFIES strings (used to cause a RecursionError)
     """
@@ -339,8 +346,23 @@ def test_large_selfies_decoding():
 
     assert decode_eq(large_selfies, expected_smiles)
 
+
+def test_radical_kekulization():
+    """Tests kekulization of aromatic systems with radicals and charges.
+    """
+    
+    assert roundtrip_eq("c1ccc[c]c1", "C1=CC=C[CH0]=C1")
+    assert roundtrip_eq("c1[c]n1(C)", "C1=[CH0]N1C")
+    assert roundtrip_eq("c1[C][n+]1(C)", "C=1[CH0][N+1]=1C")
+    assert roundtrip_eq("c1nnn[n-]1", "C1=NN=N[N-1]1")
+    assert roundtrip_eq("c1ccn[c-](C)[n+]1=O", "C1=CC=N[C-1](C)[N+1]1=O")
+    assert roundtrip_eq("c1ccs[n+]1c2ccccc2", "C=1C=CS[N+1]=1C2=CC=CC=C2")
+    assert roundtrip_eq("c1ccs[nH+]1", "C=1C=CS[NH1+1]=1")
+    
+    
 def test_novel_charged_symbols():
     """Test decoding of updated constraints for charged atoms (update in 2.2.0)."""
     assert decode_eq("[N][#C+1][#NH1][#C@H1]", "N#[C+1]")
     assert decode_eq("[O+1][=P+1][#P-1][#C@@]", "[O+1]=[P+1]=[P-1]#[C@@]")
     assert decode_eq("[=C-1][#S+1][#B]", "[C-1]#[S+1]=B")
+