diff --git a/src/kimmdy/coordinates.py b/src/kimmdy/coordinates.py
index 94daa4ad..b45c7c04 100644
--- a/src/kimmdy/coordinates.py
+++ b/src/kimmdy/coordinates.py
@@ -19,6 +19,7 @@
     Dihedral,
     DihedralType,
     ProperDihedralId,
+    ImproperDihedralId,
     MultipleDihedrals,
     Interaction,
     InteractionType,
@@ -157,8 +158,12 @@ def merge_dihedrals(
     dihedral_key: tuple[str, str, str, str],
     dihedral_a: Optional[Dihedral],
     dihedral_b: Optional[Dihedral],
-    dihedral_types_a: dict[ProperDihedralId, DihedralType],
-    dihedral_types_b: dict[ProperDihedralId, DihedralType],
+    dihedral_types_a: Union[
+        dict[ProperDihedralId, DihedralType], dict[ImproperDihedralId, DihedralType]
+    ],
+    dihedral_types_b: Union[
+        dict[ProperDihedralId, DihedralType], dict[ImproperDihedralId, DihedralType]
+    ],
     molA: MoleculeType,
     molB: MoleculeType,
     funct: str,
@@ -447,28 +452,58 @@ def merge_top_moleculetypes_slow_growth(
                 )
 
     # improper dihedrals
-    # all impropers in amber99SB ffbonded.itp have a periodicity of 2
-    # but not the ones defined in aminoacids.rtp. For now, I am assuming
-    # a periodicity of 2 in this section
+    # TODO: duplicate of proper dihedrals, could refactor
 
     keys = set(molA.improper_dihedrals.keys()) | set(molB.improper_dihedrals.keys())
     for key in keys:
-        interactionA = molA.improper_dihedrals.get(key)
-        interactionB = molB.improper_dihedrals.get(key)
+        multiple_dihedralsA = molA.improper_dihedrals.get(key)
+        multiple_dihedralsB = molB.improper_dihedrals.get(key)
 
-        if interactionA != interactionB:
-            molB.improper_dihedrals[key] = merge_dihedrals(
-                key,
-                interactionA,
-                interactionB,
-                ff.improper_dihedraltypes,
-                ff.improper_dihedraltypes,
-                molA,
-                molB,
-                "4",
-                "2",
+        if multiple_dihedralsA != multiple_dihedralsB:
+            multiple_dihedralsA = get_explicit_MultipleDihedrals(
+                key, molA, multiple_dihedralsA, ff
+            )
+            multiple_dihedralsB = get_explicit_MultipleDihedrals(
+                key, molB, multiple_dihedralsB, ff
+            )
+            keysA = (
+                set(multiple_dihedralsA.dihedrals.keys())
+                if multiple_dihedralsA
+                else set()
+            )
+            keysB = (
+                set(multiple_dihedralsB.dihedrals.keys())
+                if multiple_dihedralsB
+                else set()
             )
 
+            molB.improper_dihedrals[key] = MultipleDihedrals(*key, "9", {})
+            periodicities = keysA | keysB
+            for periodicity in periodicities:
+                assert isinstance(periodicity, str)
+                interactionA = (
+                    multiple_dihedralsA.dihedrals.get(periodicity)
+                    if multiple_dihedralsA
+                    else None
+                )
+                interactionB = (
+                    multiple_dihedralsB.dihedrals.get(periodicity)
+                    if multiple_dihedralsB
+                    else None
+                )
+
+                molB.improper_dihedrals[key].dihedrals[periodicity] = merge_dihedrals(
+                    key,
+                    interactionA,
+                    interactionB,
+                    ff.improper_dihedraltypes,
+                    ff.improper_dihedraltypes,
+                    molA,
+                    molB,
+                    "9",
+                    periodicity,
+                )
+
     # amber fix for breaking/binding atom types without LJ potential
     # breakpoint()
 
diff --git a/tests/test_coordinates.py b/tests/test_coordinates.py
index fa53e6d6..ede29545 100644
--- a/tests/test_coordinates.py
+++ b/tests/test_coordinates.py
@@ -145,7 +145,9 @@ def test_merge_prm_top(arranged_tmp_path):
     assert top_merge.bonds[("26", "27")].funct == "3"
     assert top_merge.angles[("17", "19", "20")].c3 is not None
     assert top_merge.proper_dihedrals[("15", "17", "19", "24")].dihedrals["3"].c5 == "3"
-    assert top_merge.improper_dihedrals[("17", "20", "19", "24")].c5 == "2"
+    assert (
+        top_merge.improper_dihedrals[("17", "20", "19", "24")].dihedrals["2"].c5 == "2"
+    )
     # assert one dihedral merge improper/proper