diff --git a/src/mdevaluate/checksum.py b/src/mdevaluate/checksum.py
index 21f967b..1c0d745 100755
--- a/src/mdevaluate/checksum.py
+++ b/src/mdevaluate/checksum.py
@@ -72,7 +72,7 @@ def strip_comments(source: str) -> str:
     return "\n".join([line for line in code_no_comments.splitlines() if line.strip() != ""])
 
 
-def checksum(*args, csum=None):
+def checksum(*args, csum=None, _seen=None):
     """
     Calculate a checksum of any object, by sha1 hash.
 
@@ -92,7 +92,15 @@ def checksum(*args, csum=None):
         csum = hashlib.sha1()
         csum.update(str(SALT).encode())
 
+    if _seen is None:
+        _seen = set()
+
     for arg in args:
+        obj_id = id(arg)
+        if obj_id in _seen:
+            continue
+        _seen.add(obj_id)
+
         if hasattr(arg, "__checksum__"):
             logger.debug("Checksum via __checksum__: %s", str(arg))
             csum.update(str(arg.__checksum__()).encode())
@@ -109,15 +117,15 @@ def checksum(*args, csum=None):
             for key in sorted(merged):  # deterministic ordering
                 v = merged[key]
                 if v is not arg:
-                    checksum(v, csum=csum)
+                    checksum(v, csum=csum, _seen=_seen)
         elif isinstance(arg, functools.partial):
             logger.debug("Checksum via partial for %s", str(arg))
-            checksum(arg.func, csum=csum)
+            checksum(arg.func, csum=csum, _seen=_seen)
             for x in arg.args:
-                checksum(x, csum=csum)
+                checksum(x, csum=csum, _seen=_seen)
             for k in sorted(arg.keywords.keys()):
                 csum.update(k.encode())
-                checksum(arg.keywords[k], csum=csum)
+                checksum(arg.keywords[k], csum=csum, _seen=_seen)
         elif isinstance(arg, np.ndarray):
             csum.update(arg.tobytes())
         else:
diff --git a/src/mdevaluate/distribution.py b/src/mdevaluate/distribution.py
index 847d005..3bce0f7 100644
--- a/src/mdevaluate/distribution.py
+++ b/src/mdevaluate/distribution.py
@@ -182,10 +182,10 @@ def tetrahedral_order(
     )
 
     # Connection vectors
-    neighbors_1 -= atoms
-    neighbors_2 -= atoms
-    neighbors_3 -= atoms
-    neighbors_4 -= atoms
+    neighbors_1 = pbc_diff(neighbors_1, atoms, box=atoms.box)
+    neighbors_2 = pbc_diff(neighbors_2, atoms, box=atoms.box)
+    neighbors_3 = pbc_diff(neighbors_3, atoms, box=atoms.box)
+    neighbors_4 = pbc_diff(neighbors_4, atoms, box=atoms.box)
 
     # Normed Connection vectors
     neighbors_1 /= np.linalg.norm(neighbors_1, axis=-1).reshape(-1, 1)