PythonInterface.md

@@ -15,7 +15,13 @@ A material law is defined as
 ```python
 law = MaterialLawName(matNum, Arguments)
 ```
-where the number *matNum* must be a unique integer to identify the law (when multiple laws are considered), *MaterialLawName* and its arguments *Arguments* can be found in the source code. For example, a *J2* plastic model can be defined as follows:
+where the number *matNum* must be a unique integer to identify the law (when multiple laws are considered), *MaterialLawName* and its arguments *Arguments* can be found in the source code. When using linear elements, one should use 
+```python
+law.setUseBarF(True)
+```
+to avoid volumetric locking.
+
+For example, a *J2* plastic model can be defined as follows:
 ```python
 lawnum1 = 11 # unique number of law
 E = 210E3