3. Contraction of molecular wave functions

When the separation of variables leads to a qualitatively incorrect description or a quantitatively too severe approximation, their coupling must be taken into account by contracting the wave functions. We present the method of contraction in an effective field, which not only allows us to surgically recouple certain variables according to the physics of the system under study, but also to adjust the consideration of the influence of other degrees of freedom according to the precision required. Most of the methods available in quantum chemistry software, both free and commercial, appear to be special cases of this contraction method. We will confine ourselves to describing the method for discernible degrees of freedom of the molecular degree-of-rotation-vibration type, but the method is generalizable, albeit technically more complex, to indistinguishable degrees of freedom.