diff --git a/sg.py b/sg.py
index 03e2a21..51f8691 100644
--- a/sg.py
+++ b/sg.py
@@ -29,7 +29,8 @@ def __init__(self, smdim=3):
         self.mmoi = [0, 0, 0]
         #: float: Mass-weighted radius of gyration.
         self.mwrg = None
-
+        #: float: Geometric center
+        self.gc =None
         #: int: (continuum model) Isotropy type.
         #: Isotropic (0), orthotropic (1), anisotropic (2).
         self.type = None