We want to use our knowledge of the time-dependent wave function to guide the dynamic construction of the PES. From the Born interpretation, the wave function density is an excellent measure of which parts of the PES are important to describe. In case of a onemode system the density can be the obtained directly from the norm square of the wave function. Generally, we will for multi-dimensional PESs use the reduced one-mode densities of the wave function, ρ m (q m , t), which describes the density of a mode m in the coordinate representation. The one-mode density is a function of the coordinate of the mth mode, q m , and of the time, t, and is obtained as an expectation value of the operator probing the presence of a certain mode at a point in space, δ m qm . In first quantization this operator is defined from the Dirac-delta function