We present a physically based treatment of the molecular rotational partition function for the case of the interaction of a macromolecular receptor with a small biologically active molecule. 	This leads to the proposal of a new substituent parameter that we call the Orientational Parameter. Its physical interpretation is that it gives an account of the substituent s influence on the percentage of molecules achieving the correct orientation to interact with the receptor. 	Finally we propose a universal way to calculate its values, avoiding the need of building long tables. A comparison between the old and new ways to calculate it is presented. We provide here an example to show shown its usefulness in QSAR studies