add test for rigid_id assignment

gmso/tests/test_convert_mbuild.py

@@ -259,3 +259,14 @@ def test_nontop_level_compound(self, mb_ethane):
         cpd.add(mb_ethane)
         with pytest.raises(AssertionError):
             from_mbuild(mb_ethane)
+
+    def test_rigid_ids(self, mb_ethane):
+        box = mb.fill_box(mb_ethane, n_compounds=3, box=[2, 2, 2])
+        for i, child in enumerate(box.children):
+            for p in child.particles():
+                p.rigid_id = i
+
+        top = from_mbuild(box)
+        assert set([site.rigid_id for site in top.sites]) == {0, 1, 2}
+        for particle, site in zip(box.particles(), top.sites):
+            assert particle.rigid_id == site.rigid_id