Expanded search for improper_types

gmso/core/forcefield.py

@@ -1,4 +1,5 @@
 """Module for working with GMSO forcefields."""
+import itertools
 import warnings
 from collections import ChainMap
 from typing import Iterable
@@ -409,14 +410,18 @@ def _get_improper_type(self, atom_types, warn=False):
             )
 
         forward = FF_TOKENS_SEPARATOR.join(atom_types)
-        reverse = FF_TOKENS_SEPARATOR.join(
-            [atom_types[0], atom_types[2], atom_types[1], atom_types[3]]
-        )
+        equivalent = [
+            FF_TOKENS_SEPARATOR.join(
+                [atom_types[0], atom_types[i], atom_types[j], atom_types[k]]
+            )
+            for (i, j, k) in itertools.permutations((1, 2, 3), 3)
+        ]
 
         if forward in self.improper_types:
             return self.improper_types[forward]
-        if reverse in self.improper_types:
-            return self.improper_types[reverse]
+        for eq in equivalent:
+            if eq in self.improper_types:
+                return self.improper_types[eq]
 
         match = None
         for i in range(1, 5):

gmso/tests/test_forcefield.py

@@ -5,6 +5,7 @@
 from sympy import sympify
 
 from gmso.core.forcefield import ForceField
+from gmso.core.improper_type import ImproperType
 from gmso.exceptions import (
     ForceFieldParseError,
     MissingAtomTypesError,
@@ -596,3 +597,19 @@ def test_non_element_types(self, non_element_ff, opls_ethane_foyer):
             ).definition
             == "[_CH2;X2]([_CH3,_CH2])[_CH3,_CH2]"
         )
+
+    def test_forcefield_get_impropers_combinations(self):
+        ff_with_impropers = ForceField()
+        ff_with_impropers.name = "imp_ff"
+        ff_with_impropers.improper_types = {
+            "CT~CT~HC~HC": ImproperType(name="imp1"),
+            "CT~HC~HC~HC": ImproperType(name="imp2"),
+        }
+        imp1 = ff_with_impropers.get_potential(
+            "improper_type", ["CT", "HC", "HC", "CT"]
+        )
+        imp2 = ff_with_impropers.get_potential(
+            "improper_type", ["CT", "HC", "CT", "HC"]
+        )
+        assert imp1.name == imp2.name
+        assert imp1 is imp2