diff --git a/pyxtal/symmetry.py b/pyxtal/symmetry.py
index 01cd55b2..fecdf236 100644
--- a/pyxtal/symmetry.py
+++ b/pyxtal/symmetry.py
@@ -390,22 +390,26 @@ def is_valid_combination(self, sites):
                     return False
         return True
 
-    def list_wyckoff_combinations(self, numIons, quick=False, max_wp=None, min_wp=None, Nmax=10000000):
+    def list_wyckoff_combinations(self, numIons, quick=False, numWp=(None, None), Nmax=10000000):
         """
         List all possible wyckoff combinations for the given formula. Note this
         is really designed for a light weight calculation. If the solution space
         is big, set quick as True.
 
         Args:
-            numIons: [12, 8]
-            quick: Boolean, quickly generate some solutions
+            numIons (list): [12, 8]
+            quick ()Boolean): quickly generate some solutions
+            numWp (tuple): (min_wp, max_wp)
+            Nmax: maximumly allowed combinations
 
         Returns:
             Combinations: list of possible sites
             has_freedom: list of boolean numbers
+            indices: list of wp indices
         """
 
         numIons = np.array(numIons)
+        (min_wp, max_wp) = numWp
         # Must be greater than the number of smallest wp multiplicity
         if numIons.min() < self[-1].multiplicity:
             return [], [], []
diff --git a/pyxtal/util.py b/pyxtal/util.py
index 69ec5182..f6abe1f8 100644
--- a/pyxtal/util.py
+++ b/pyxtal/util.py
@@ -529,8 +529,7 @@ def generate_wp_lib(spg_list, composition,
         count = 0
         for i in range(max_fu, min_fu-1, -1):
             letters, _, wp_ids = g.list_wyckoff_combinations(
-                    composition*i, max_wp=max_wp,
-                    min_wp=min_wp, Nmax=100000)
+                    composition*i, num_wp=(min_wp, max_wp), Nmax=100000)
             for j, wp in enumerate(wp_ids):
                 wp_dofs = 0
                 num = 0