Now should work for multi-chain. But it is untested.

Also fixed important bug: atom names were padded

openfold/np/protein.py

@@ -403,7 +403,6 @@ def to_modelcif(prot: Protein) -> str:
     """
 
     restypes = residue_constants.restypes + ["X"]
-    res_1to3 = lambda r: residue_constants.restype_1to3.get(restypes[r], "UNK")
     atom_types = residue_constants.atom_types
 
     atom_mask = prot.atom_mask
@@ -413,71 +412,67 @@ def to_modelcif(prot: Protein) -> str:
     b_factors = prot.b_factors
     chain_index = prot.chain_index
 
+    n = aatype.shape[0]
+    if chain_index is None:
+        chain_index = [0 for i in range(n)]
+
     system = modelcif.System(title='OpenFold prediction')
 
-    # sequence into entity
-    n = aatype.shape[0]
-    seq = [restypes[aatype[i]] for i in range(n)]
-    model_e = modelcif.Entity("".join(seq), description='Model subunit')
+    # Finding chains and creating entities
+    seqs = {}
+    seq = []
+    last_chain_idx = None
+    for i in range(n):
+        if last_chain_idx is not None and last_chain_idx != chain_index[i]:
+            seqs[last_chain_idx] = seq
+            seq = []
+        seq.append(restypes[aatype[i]])
+        last_chain_idx = chain_index[i]
+    # finally add the last chain
+    if last_chain_idx not in seqs:
+        seqs[last_chain_idx] = seq
+
+    # now reduce sequences to unique ones (note this won't work if different asyms have different unmodelled regions)
+    unique_seqs = {}
+    for chain_idx in seqs.keys():
+        seq = "".join(seqs[chain_idx])
+        if seq in unique_seqs:
+            unique_seqs[seq].append(chain_idx)
+        else:
+            unique_seqs[seq] = [chain_idx]
 
-    # Define the model assembly
-    asymA = modelcif.AsymUnit(model_e, details='Model subunit A', id='A')
-    modeled_assembly = modelcif.Assembly((asymA,), name='Modeled assembly')
+    # adding 1 entity per unique sequence
+    entities_map = {}
+    for key, value in unique_seqs.items():
+        model_e = modelcif.Entity("".join(key), description='Model subunit')
+        for chain_idx in value:
+            entities_map[chain_idx] = model_e
 
-    class MyModel(modelcif.model.HomologyModel):
-        asym_unit_map = {'A': asymA}
+    chain_tags = string.ascii_uppercase
+    asym_unit_map = {}
+    for chain_idx in set(chain_index):
+        # Define the model assembly
+        chain_id = chain_tags[chain_idx]
+        asym = modelcif.AsymUnit(entities_map[chain_idx], details='Model subunit %s' % chain_id, id=chain_id)
+        asym_unit_map[chain_idx] = asym
+    modeled_assembly = modelcif.Assembly(asym_unit_map.values(), name='Modeled assembly')
 
+    class MyModel(modelcif.model.HomologyModel):
         def get_atoms(self):
-
-            prev_chain_index = 0
-            chain_tags = string.ascii_uppercase
             # Add all atom sites.
             for i in range(n):
-                res_name_3 = res_1to3(aatype[i])
                 for atom_name, pos, mask, b_factor in zip(
                         atom_types, atom_positions[i], atom_mask[i], b_factors[i]
                 ):
                     if mask < 0.5:
                         continue
-
-                    name = atom_name if len(atom_name) == 4 else f" {atom_name}"
                     element = atom_name[0]  # Protein supports only C, N, O, S, this works.
-
-                    chain_tag = "A"
-                    if chain_index is not None:
-                        chain_tag = chain_tags[chain_index[i]]
-
-                    # TODO check that residue indices are ok
-                    # TODO how to set residue types? do they come from the entity sequence set above?
                     yield modelcif.model.Atom(
-                        asym_unit=asymA, type_symbol=element,
-                        seq_id=residue_index[i], atom_id=name,
+                        asym_unit=asym_unit_map[chain_index[i]], type_symbol=element,
+                        seq_id=residue_index[i], atom_id=atom_name,
                         x=pos[0], y=pos[1], z=pos[2],
                         het=False, biso=b_factor, occupancy=1.00)
 
-                # TODO multiple chains
-                # should_terminate = (i == n - 1)
-                # if(chain_index is not None):
-                #     if(i != n - 1 and chain_index[i + 1] != prev_chain_index):
-                #         should_terminate = True
-                #         prev_chain_index = chain_index[i + 1]
-                #
-                # if(should_terminate):
-                #     # Close the chain.
-                #     chain_end = "TER"
-                #     chain_termination_line = (
-                #         f"{chain_end:<6}{atom_index:>5}      "
-                #         f"{res_1to3(aatype[i]):>3} "
-                #         f"{chain_tag:>1}{residue_index[i]:>4}"
-                #     )
-                #     pdb_lines.append(chain_termination_line)
-                #     atom_index += 1
-                #
-                #     if(i != n - 1):
-                #         # "prev" is a misnomer here. This happens at the beginning of
-                #         # each new chain.
-                #         pdb_lines.extend(get_pdb_headers(prot, prev_chain_index))
-
     # Add the model and modeling protocol to the file and write them out:
     model = MyModel(assembly=modeled_assembly, name='Best scoring model')
 
@@ -548,9 +543,15 @@ def from_prediction(
 
 
 if __name__ == "__main__":
-    with open('/home/jose/Downloads/171l.pdb', 'r') as file:
+    pdb_file = '/home/jose/Downloads/171l.pdb'
+    # pdb_file = '/home/jose/Downloads/2trx.pdb'
+    cif_file = '/home/jose/test.cif'
+
+    with open(pdb_file, 'r') as file:
         pdbstr = file.read()
 
-    prot = from_pdb_string(pdbstr)
+    prot = from_pdb_string(pdbstr, "A")
     cifstr = to_modelcif(prot)
-    print(cifstr)
+    print(cifstr)
\ No newline at end of file
+    with open(cif_file, 'w') as fw:
+        fw.write(cifstr)