diff --git a/graphiti_core/errors.py b/graphiti_core/errors.py
index 58a8ee3b..cdf74e95 100644
--- a/graphiti_core/errors.py
+++ b/graphiti_core/errors.py
@@ -35,6 +35,14 @@ class GroupsEdgesNotFoundError(GraphitiError):
         super().__init__(self.message)
 
 
+class GroupsNodesNotFoundError(GraphitiError):
+    """Raised when no nodes are found for a list of group ids."""
+
+    def __init__(self, group_ids: list[str]):
+        self.message = f'no nodes found for group ids {group_ids}'
+        super().__init__(self.message)
+
+
 class NodeNotFoundError(GraphitiError):
     """Raised when a node is not found."""
 
diff --git a/graphiti_core/search/search_utils.py b/graphiti_core/search/search_utils.py
index d818e090..c75dabd8 100644
--- a/graphiti_core/search/search_utils.py
+++ b/graphiti_core/search/search_utils.py
@@ -589,7 +589,7 @@ def maximal_marginal_relevance(
     candidates_with_mmr: list[tuple[str, float]] = []
     for candidate in candidates:
         max_sim = max([np.dot(normalize_l2(candidate[1]), normalize_l2(c[1])) for c in candidates])
-        mmr = mmr_lambda * np.dot(candidate[1], query_vector) + (1 - mmr_lambda) * max_sim
+        mmr = mmr_lambda * np.dot(candidate[1], query_vector) - (1 - mmr_lambda) * max_sim
         candidates_with_mmr.append((candidate[0], mmr))
 
     candidates_with_mmr.sort(reverse=True, key=lambda c: c[1])