In the first paper of this series, we will describe a method, called effective group potentials (EGP), aimed at simplifying molecular ab initio calculations for large systems involving bulky ligands as long as these ligands can be supposed to play the role of spectator groups. This method should be applicable to all types of bondings between active and the spectator parts. The different steps used in the definition of the EGP are closely related to those used for atomic effective core potentials (ECP) which are commonly used with great success in ab initio calculations involving heavy atoms.