diff --git a/src/mcsce/cli.py b/src/mcsce/cli.py
index f9974f4..1adaca4 100644
--- a/src/mcsce/cli.py
+++ b/src/mcsce/cli.py
@@ -44,6 +44,18 @@ def read_structure_and_check(file_name, retain_idx=[], verbose=False):
     """
     s = Structure(file_name)
     s.build()
+    his_id = [str(resid) for resid, res_type in zip(s.residues, s.residue_types) if res_type == "HIS"]
+    if len(his_id) > 0:
+        message = f"WARNING! Undefined histidine protonation status found for structure [{file_name}]:\n"
+        message += " ".join(his_id)
+        message += "\nThese residues have been modified to HIP\n"
+        res_labels = s.res_labels
+        res_labels[res_labels == "HIS"] = "HIP"
+        s.res_labels = res_labels
+        if verbose:
+            print(message)
+        else:
+            print("HIS residues modified to HIP\n")
     missing_backbone_atoms = s.check_backbone_atom_completeness()
     if len(missing_backbone_atoms) > 0:
         message = f"WARNING! These atoms are missing from the current backbone structure [{file_name}]:"
@@ -133,7 +145,7 @@ def main(input_structure, n_conf, n_worker, output_dir, logfile, mode, fix, batc
                 return
             # remove added sidechains in sections to be processed
             s = s.remove_side_chains(fix_idxs)
-       
+      
         initialize_func_calc(partial(prepare_energy_function, batch_size=batch_size,
                    forcefield=ff_obj, terms=["lj", "clash", "coulomb"]),
                    structure=s, retain_idxs=fix_idxs)
diff --git a/src/mcsce/core/side_chain_builder.py b/src/mcsce/core/side_chain_builder.py
index e7ffc92..a7755ff 100644
--- a/src/mcsce/core/side_chain_builder.py
+++ b/src/mcsce/core/side_chain_builder.py
@@ -74,23 +74,31 @@ def initialize_func_calc(efunc_creator, aa_seq=None, structure=None, retain_idxs
         # extract amino acid sequence from structure object
         aa_seq = structure.residue_types
     structure = deepcopy(structure)
+    chain_ids = structure.residue_chains
 
     sidechain_placeholders.append(deepcopy(structure))
+    n_terms, c_terms = structure.get_terminal_res_atom_arr()
     energy_calculators.append(efunc_creator(structure.atom_labels, 
                                             structure.res_nums,
-                                            structure.res_labels))
-    for idx, resname in tqdm(enumerate(aa_seq), total=len(aa_seq)):
+                                            structure.res_labels,
+                                            n_terms,
+                                            c_terms))
+                                            
+    for idx, resname, chain_id in tqdm(zip(range(len(aa_seq)), aa_seq, chain_ids), total=len(aa_seq)):
         if idx + structure.res_nums[0] not in retain_idxs: 
             template = sidechain_templates[resname]
-            structure.add_side_chain(idx + structure.res_nums[0], template)
+            structure.add_side_chain(idx + structure.res_nums[0], template, chain_id)
         sidechain_placeholders.append(deepcopy(structure))
         
         if resname not in ["GLY", "ALA"] and idx + structure.res_nums[0] not in retain_idxs:
             n_sidechain_atoms = len(template[1])
             all_indices = np.arange(len(structure.atom_labels))
+            n_terms, c_terms = structure.get_terminal_res_atom_arr()
             energy_func = efunc_creator(structure.atom_labels, 
                                         structure.res_nums,
                                         structure.res_labels,
+                                        n_terms,
+                                        c_terms,
                                         partial_indices=[all_indices[-n_sidechain_atoms:],
                                                          all_indices[:-n_sidechain_atoms]])
             energy_calculators.append(energy_func)
diff --git a/src/mcsce/libs/libenergy.py b/src/mcsce/libs/libenergy.py
index c85abd1..0999ec5 100644
--- a/src/mcsce/libs/libenergy.py
+++ b/src/mcsce/libs/libenergy.py
@@ -28,6 +28,8 @@ def prepare_energy_function(
         atom_labels,
         residue_numbers,
         residue_labels,
+        N_terminal_indicators,
+        C_terminal_indicators,
         forcefield,
         batch_size=16,
         partial_indices=None,
@@ -162,6 +164,8 @@ def prepare_energy_function(
             new_indices,
             old_indices,
             forcefield.forcefield,
+            N_terminal_indicators,
+            C_terminal_indicators,
             )
             
         # 0.2 as 0.4
@@ -387,7 +391,7 @@ def create_bonds_apart_mask_for_ij_pairs_old(
         residue_labels_ij_gen,
         residue_numbers_ij_gen,
         atom_labels_ij_gen,
-        bonds_intra,
+        bonds_atom_labelsintra,
         bonds_inter,
         )
 
@@ -407,15 +411,18 @@ def create_LJ_params_raw(
         new_indices,
         old_indices,
         force_field,
+        n_terminal_indicators,
+        c_terminal_indicators,
         ):
     """Create ACOEFF and BCOEFF parameters.
     Borrowed from IDP Conformer Generator package (https://github.com/julie-forman-kay-lab/IDPConformerGenerator) developed by Joao M. C. Teixeira"""
+
     sigmas_ii_new = extract_ff_params_for_seq(
         atom_labels[new_indices],
         residue_numbers[new_indices],
         residue_labels[new_indices],
-        min(residue_numbers),
-        max(residue_numbers),
+        n_terminal_indicators[new_indices],
+        c_terminal_indicators[new_indices],
         force_field,
         'sigma',
         )
@@ -424,8 +431,8 @@ def create_LJ_params_raw(
         atom_labels[old_indices],
         residue_numbers[old_indices],
         residue_labels[old_indices],
-        min(residue_numbers),
-        max(residue_numbers),
+        n_terminal_indicators[old_indices],
+        c_terminal_indicators[old_indices],
         force_field,
         'sigma',
         )
@@ -434,8 +441,8 @@ def create_LJ_params_raw(
         atom_labels[new_indices],
         residue_numbers[new_indices],
         residue_labels[new_indices],
-        min(residue_numbers),
-        max(residue_numbers),
+        n_terminal_indicators[new_indices],
+        c_terminal_indicators[new_indices],
         force_field,
         'epsilon',
         )
@@ -444,8 +451,8 @@ def create_LJ_params_raw(
         atom_labels[old_indices],
         residue_numbers[old_indices],
         residue_labels[old_indices],
-        min(residue_numbers),
-        max(residue_numbers),
+        n_terminal_indicators[old_indices],
+        c_terminal_indicators[old_indices],
         force_field,
         'epsilon',
         )
@@ -463,6 +470,7 @@ def create_LJ_params_raw(
 
     # mixing rules
     epsilons_ij = epsilons_ij_pre ** 0.5
+    
     # mixing + nm to Angstrom converstion
     # / 2 and * 10
     sigmas_ij = sigmas_ij_pre * 5
diff --git a/src/mcsce/libs/libparse.py b/src/mcsce/libs/libparse.py
index 9414baf..13cbfb2 100644
--- a/src/mcsce/libs/libparse.py
+++ b/src/mcsce/libs/libparse.py
@@ -415,15 +415,16 @@ def extract_ff_params_for_seq(
     params_l = []
     params_append = params_l.append
 
-    zipit = zip(atom_labels, residue_numbers, residue_labels)
-    for atom_name, res_num, res_label in zipit:
-        #print(atom_name, res_num, res_label)
+    zipit = zip(atom_labels, residue_numbers, residue_labels, n_terminal_idx, c_terminal_idx)
+    for atom_name, res_num, res_label, is_nterm, is_cterm in zipit:
+
         # adds N and C to the terminal residues
-        if res_num == n_terminal_idx:
+
+        if is_nterm:
             res = 'N' + res_label
             assert res.isupper() and len(res) == 4, res
 
-        elif res_num == c_terminal_idx:
+        elif is_cterm:
             res = 'C' + res_label
             assert res.isupper() and len(res) == 4, res
         else:
diff --git a/src/mcsce/libs/libstructure.py b/src/mcsce/libs/libstructure.py
index 0d65b6d..44f53f3 100644
--- a/src/mcsce/libs/libstructure.py
+++ b/src/mcsce/libs/libstructure.py
@@ -158,6 +158,29 @@ def coords(self):
     def coords(self, coords):
         self.data_array[:, cols_coords] = \
             np.round(coords, decimals=3).astype('<U8')
+        
+
+    def get_terminal_res_atom_arr(self):
+        """Return two boolean arrays, each one equal to the total length of the structure,
+          indicating whether atoms are in N/C terminals."""
+        c, rs, rn = col_chainID, col_resSeq, col_resName
+
+        chains = defaultdict(dict)
+        for row in self.filtered_atoms:
+            chains[row[c]].setdefault(row[rs], row[rn])
+
+        N_term_indicator = np.zeros(len(self.filtered_atoms), dtype=bool)
+        C_term_indicator = np.zeros(len(self.filtered_atoms), dtype=bool)
+        for chain_name, chain_residues in chains.items():
+            N_res_id = list(chain_residues.keys())[0]
+            N_res_chain_indicator = (self.filtered_atoms[:, c] == chain_name) & (self.filtered_atoms[:, rs] == N_res_id)
+            N_term_indicator = N_term_indicator | N_res_chain_indicator
+
+            C_res_id = list(chain_residues.keys())[-1]
+            C_res_chain_indicator = (self.filtered_atoms[:, c] == chain_name) & (self.filtered_atoms[:, rs] == C_res_id)
+            C_term_indicator = C_term_indicator | C_res_chain_indicator
+
+        return N_term_indicator, C_term_indicator
 
 
 
@@ -196,7 +219,23 @@ def residue_types(self):
         chains = defaultdict(dict)
         for row in self.filtered_atoms:
             chains[row[c]].setdefault(row[rs], row[rn])
-        return list(list(chains.values())[0].values())
+        restypes = []
+        for chain_residues in chains.values():
+            restypes.extend(list(chain_residues.values()))
+        return restypes
+    
+    @property
+    def residue_chains(self):
+        c, rs, rn = col_chainID, col_resSeq, col_resName
+
+        chains = defaultdict(dict)
+        for row in self.filtered_atoms:
+            chains[row[c]].setdefault(row[rs], row[rn])
+
+        chain_ids = []
+        for chain_id in chains:
+            chain_ids.extend([chain_id] * len(chains[chain_id]))
+        return chain_ids
 
     @property
     def filtered_residues(self):
@@ -215,15 +254,15 @@ def residues(self):
 
     @property
     def atom_labels(self):
-        return self.data_array[:, col_name]
+        return self.filtered_atoms[:, col_name]
 
     @property
     def res_nums(self):
-        return self.data_array[:, col_resSeq].astype(int)
+        return self.filtered_atoms[:, col_resSeq].astype(int)
 
     @property
     def res_labels(self):
-        return self.data_array[:, col_resName]
+        return self.filtered_atoms[:, col_resName]
 
     @residues.setter
     def residues(self, residue_idx):
@@ -235,6 +274,10 @@ def residues(self, residue_idx):
         """
         self.data_array[:, col_resSeq] = str(residue_idx)
 
+    @res_labels.setter
+    def res_labels(self, res_labels):
+        self.data_array[:, col_resName] = res_labels
+
     def pop_last_filter(self):
         """Pop last filter."""
         self._filters.pop()
@@ -412,16 +455,17 @@ def check_backbone_atom_completeness(self):
         Run check of backbone atom completeness and return a list containing all missing atoms from expected backbone atom list
         '''
         all_residue_atoms = {}
-        for atom_label, res_num, res_label in zip(self.atom_labels, self.res_nums, self.res_labels):
+        N_res_indicators, C_res_indicators = self.get_terminal_res_atom_arr()
+        for atom_label, res_num, res_label, is_N_res, is_C_res in \
+            zip(self.atom_labels, self.res_nums, self.res_labels, N_res_indicators, C_res_indicators):
             if res_num not in all_residue_atoms:
-                all_residue_atoms[res_num] = {"label": res_label, "atoms": [atom_label]}
+                all_residue_atoms[res_num] = {"label": res_label, "atoms": [atom_label], "is_N_res": is_N_res, "is_C_res": is_C_res}
             else:
                 all_residue_atoms[res_num]["atoms"].append(atom_label)
-        n_term_idx = min(all_residue_atoms)
-        c_term_idx = max(all_residue_atoms)
+
         missing_atoms = []
         for idx in all_residue_atoms:
-            if idx == n_term_idx:
+            if all_residue_atoms[idx]["is_N_res"]:
                 expected_atoms = ["N", "CA", "C", "O", "H1", "H2"]
                 if all_residue_atoms[idx]["label"] not in ["PRO", "HYP"]:
                     expected_atoms.append("H3")
@@ -429,7 +473,7 @@ def check_backbone_atom_completeness(self):
                 expected_atoms = ["N", "CA", "C", "O"]
                 if all_residue_atoms[idx]["label"] not in ["PRO", "HYP"]:
                     expected_atoms.append("H")
-                if idx == c_term_idx:
+                if all_residue_atoms[idx]["is_C_res"]:
                     expected_atoms.append("OXT")
             residue_missing_atom = [item for item in expected_atoms \
                 if item not in all_residue_atoms[idx]["atoms"]]
@@ -451,7 +495,7 @@ def remove_side_chains(self, retain_idxs=[]):
         copied_structure._data_array = copied_structure.data_array[retained_atoms_filter]
         return copied_structure #, None if np.all(retained_atoms_filter) else retained_atoms_filter
 
-    def add_side_chain(self, res_idx, sidechain_template):
+    def add_side_chain(self, res_idx, sidechain_template, chain_id='A'):
         template_structure, sc_atoms = sidechain_template
         self.add_filter_resnum(res_idx)
         N_CA_C_coords = self.get_sorted_minimal_backbone_coords(filtered=True)
@@ -459,11 +503,13 @@ def add_side_chain(self, res_idx, sidechain_template):
         sidechain_data_arr = template_structure.data_array.copy()
         sidechain_data_arr[:, cols_coords] = np.round(sc_all_atom_coords, decimals=3).astype('<U8')
         sidechain_data_arr[:, col_resSeq] = str(res_idx)
+
         # conform to backbone residue labels but conform to sidechain records
         res_mask = (self.data_array[:, col_resSeq].astype(int) == res_idx)
         self.data_array[res_mask, col_record] = sidechain_data_arr[0, col_record]
         sidechain_data_arr[:, col_segid] = str(self.filtered_atoms[0, col_segid])
-        sidechain_data_arr[:, col_chainID] = str(self.filtered_atoms[0, col_chainID])
+        sidechain_data_arr[:, col_chainID] = chain_id
+
         self.pop_last_filter()
         self._data_array = np.concatenate([self.data_array, sidechain_data_arr[sc_atoms]])