in the first paper of this series, we will describe a method, called effect
ive group potentials (EGP), aimed at simplifying molecular ab initio calcul
ations for large systems involving bulky ligands as long as these ligands c
an he 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 ECP 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.