WOS:000295110300016 ; International audience ; The determination and the characterization of important points of a potential energy surface can be carried out routinely using any molecular modelling software based on either a quantum mechanical (QM) or a classical (molecular mechanics, MM) description of the particle interactions. However the coupling of the QM and MM descriptions (QM/MM) gives rise to severe efficiency limitations during a geometry optimization of the whole system, especially when both subsystems are coupled electrostatically. This work introduces two simple improvements, mainly developed in the framework of the electrostatic potential fitted method to ensure the uniqueness of the QM/MM potential energy surface. The first one aims to improve the approximate Hessian matrix used in the