diff --git a/doc/src/JPG/third_order_force_constant.png b/doc/src/JPG/third_order_force_constant.png
deleted file mode 100644
index f6171ccf09..0000000000
Binary files a/doc/src/JPG/third_order_force_constant.png and /dev/null differ
diff --git a/doc/src/third_order.rst b/doc/src/third_order.rst
index b163844365..2635c92f84 100644
--- a/doc/src/third_order.rst
+++ b/doc/src/third_order.rst
@@ -39,8 +39,9 @@ Description
 
 Calculate the third order force constant tensor by finite difference of the selected group,
 
-.. image:: JPG/third_order_force_constant.png
-   :align: center
+.. math::
+
+    \Phi^{\alpha\beta\gamma}_{ijk} = \frac{\partial^3 U}{\partial x_{i,\alpha} \partial x_{j,\beta} \partial x_{k, \gamma}}
 
 where Phi is the third order force constant tensor.