diff --git a/mddatasetbuilder/datasetbuilder.py b/mddatasetbuilder/datasetbuilder.py
index bc21402..e3d0e19 100644
--- a/mddatasetbuilder/datasetbuilder.py
+++ b/mddatasetbuilder/datasetbuilder.py
@@ -393,7 +393,7 @@ def detect_multiplicity(symbols):
         """
         # currently only support charge=0
         # oxygen -> 3
-        if np.count_nonzero(symbols == ["O"]) == 2 and symbols.size == 2:
+        if symbols == ["O", "O"]:
             return 3
         # calculates the total number of electrons, assumes they are paired as much as possible
         n_total = sum([atomic_numbers[s] for s in symbols])