Flexible monomer formulation for non-rigid systems

In this Letter, the general and rigorous approach for deriving adiabatically constrained kinetic energy operators of Gatti et al. is reformulated and simplified. Emphasis is placed on weakly bound systems such as van der Waals polymers, molecules bound to a surface or located inside a rare gas matrix since an adiabatic approximation might turn out to be very relevant for them. This allows us to recast the flexible adiabatic monomer formulation into that for the rigid case.