diff --git a/src/trexio_tools/converters/convert_from.py b/src/trexio_tools/converters/convert_from.py
index e1ffb0b..4113c17 100644
--- a/src/trexio_tools/converters/convert_from.py
+++ b/src/trexio_tools/converters/convert_from.py
@@ -406,9 +406,9 @@ def run_resultsFile(trexio_file, filename, motype=None):
         det_list = []
         for i in range(determinant_num):
             det_tmp      = []
-            orb_list_up  = [ orb+1 for orb in res.determinants[i].get("alpha") ]
+            orb_list_up  = [ orb for orb in res.determinants[i].get("alpha") ]
             det_tmp     += trexio_det.to_determinant_list(orb_list_up, int64_num)
-            orb_list_dn  = [ orb+1 for orb in res.determinants[i].get("beta") ]
+            orb_list_dn  = [ orb for orb in res.determinants[i].get("beta") ]
             det_tmp     += trexio_det.to_determinant_list(orb_list_dn, int64_num)
 
             det_list.append(det_tmp)
diff --git a/src/trexio_tools/group_tools/determinant.py b/src/trexio_tools/group_tools/determinant.py
index a2ead69..458d8d9 100644
--- a/src/trexio_tools/group_tools/determinant.py
+++ b/src/trexio_tools/group_tools/determinant.py
@@ -1,14 +1,28 @@
 #!/usr/bin/env python3
 
 def to_determinant_list(orbital_list: list, int64_num: int) -> list:
-    """Convert an input orbital_list into a set of bit fields."""
+    """
+    Convert a list of occupied orbitals from the `orbital_list`
+    into a list of Slater determinants (in their bit string representation).
+
+    Orbitals in the `orbital_list` should be 0-based, namely the lowest orbital has index 0, not 1.
+
+    int64_num is the number of 64-bit integers needed to represent a Slater determinant bit string.
+    It depends on the number of molecular orbitals as follows: int64_num = int((mo_num-1)/64) + 1
+    """
+
+    if not isinstance(orbital_list, list):
+        raise TypeError(f"orbital_list should be a list, not {type(orbital_list)}")
 
     det_list = []
     bitfield = 0
     shift    = 0
 
+    # since orbital indices are 0-based but the code below works for 1-based --> increment the input indices by +1
+    orb_list_upshifted = [ orb+1 for orb in orbital_list]
+
     # orbital list has to be sorted in increasing order for the bitfields to be set correctly
-    orb_list_sorted = sorted(orbital_list)
+    orb_list_sorted = sorted(orb_list_upshifted)
 
     for orb in orb_list_sorted:
 
@@ -23,15 +37,15 @@ def to_determinant_list(orbital_list: list, int64_num: int) -> list:
         shift  = modulo*64
         bitfield |= (1 << (orb-shift))
 
-    # this removes the 0 bit from the beginning of the bitfield 
+    # this removes the 0 bit from the beginning of the bitfield
     bitfield = bitfield >> 1
     det_list.append(bitfield)
     #print('Popcounts: ', [bin(d).count('1') for d in det_list)
     #print('Bitfields: ', [bin(d) for d in det_list])
-    
+
     bitfield_num = len(det_list)
     if bitfield_num > int64_num:
-        raise Exception(f'Number of bitfields {bitfield_num} cannot be more than the int64_num {int64_num}.') 
+        raise Exception(f'Number of bitfields {bitfield_num} cannot be more than the int64_num {int64_num}.')
     if bitfield_num < int64_num:
         for _ in range(int64_num - bitfield_num):
             print("Appending an empty bitfield.")