diff --git a/doc/conf.py b/doc/conf.py
index 235eb066..32fcec8b 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -17,7 +17,7 @@
 
 sys.path.insert(0, os.path.abspath("../"))
 # List of modules to mock
-MOCK_MODULES = ['torchani', 'lammps']
+MOCK_MODULES = ['torchani', 'lammps', 'pyshtools']
 
 class Mock(MagicMock):
     @classmethod
diff --git a/pyxtal/db.py b/pyxtal/db.py
index f44d3f19..52d89006 100644
--- a/pyxtal/db.py
+++ b/pyxtal/db.py
@@ -824,12 +824,9 @@ def clean_structures(self, ids=(None, None), dtol=2e-3, etol=1e-3, criteria=None
 
     def clean_structures_pmg(self, ids=(None, None), min_id=None, dtol=5e-2, criteria=None):
         """
-        Clean up the db by removing the duplicate structures
-        Here we check the follow criteria
-            - same density
-            - pymatgen check
-
-        criteria should look like the following,
+        Clean up the db by removing the duplicate structures.
+        Here we check the follow criteria same density and pymatgen matcher.
+        The criteria should look like the following,
         {'CN': {'C': 3},
          'cutoff': 1.8,
          'MAX_energy': -8.00,