diff --git a/src/tequila/quantumchemistry/qc_base.py b/src/tequila/quantumchemistry/qc_base.py
index 9c3342fb..a70d5677 100644
--- a/src/tequila/quantumchemistry/qc_base.py
+++ b/src/tequila/quantumchemistry/qc_base.py
@@ -684,7 +684,7 @@ def orthonormalize_basis_orbitals(self):
         # backward compatibility
         return self.use_native_orbitals()
 
-    def use_native_orbitals(self, inplace=False, core: list = None, *args, **kwargs):
+    def use_native_orbitals(self, inplace=False, core: list = [], *args, **kwargs):
         """
         Parameters
         ----------
@@ -786,23 +786,39 @@ def get_core(active):
             co.sort()
             return co
 
+        def active_to_active(active):
+            '''
+            translates active indices from canonical/the original basis to the native coeffs
+            '''
+            ov = numpy.zeros(shape=(len(self.integral_manager.orbitals)))
+            for i in active:
+                for j in range(len(d)):
+                    ov[j] += numpy.abs(inner(c.T[i], d.T[j], s))
+            act = []
+            for i in range(len(active)):
+                idx = numpy.argmax(ov)
+                act.append(idx)
+                ov[idx] = 0.
+            act.sort()
+            return act
+        
         active = None
-        if not self.integral_manager.active_space_is_trivial() and core is None:
-            core = [i.idx_total for i in self.integral_manager.orbitals if i.idx is None]
         if "active" in kwargs:
             active = kwargs["active"]
             kwargs.pop("active")
-            if core is None:
+            if not len(core):
                 core = get_core(active)
         else:
-            if active is None:
-                if core is None:
+            if not len(core):
+                if not self.integral_manager.active_space_is_trivial():
+                    active = [i.idx_total for i in self.integral_manager.orbitals if i.idx is not None]
+                    active = active_to_active(active)
+                    core = [i.idx_total for i in self.integral_manager.orbitals if i.idx is None]
+                else:
                     core = []
                     active = [i for i in range(len(self.integral_manager.orbitals))]
-                else:
-                    if isinstance(core, int):
-                        core = [core]
-                    active = get_active(core)
+            else:
+                active = get_active(core)
         assert len(active) + len(core) == len(self.integral_manager.orbitals)
         to_active = [i for i in range(len(self.integral_manager.orbitals)) if i not in core]
         to_active = {active[i]: to_active[i] for i in range(len(active))}