Most of the quantum-chemical calculations have been performed using the density functional theory approximation PBE/def2-SVP (in some cases paired with D3BJ empirical dispersion corrections) implemented in the ORCA 5 package. In cases where a better agreement with experimental data was required, the composite method PBEh-3c has been utilized. Vibrational corrections to rotational constants within VPT2 theory were computed by the Gaussian 16 package. Details on methods and settings are provided in ESI. To illustrate the general idea of WS and SWS in application to molecular vibrations, we have done calculations for a simple diatomic model, the molecular cation H + 2 . Here we focus on the interatomic pair distribution function, as this provides a general representation of the molecular ensemble, from which other observables can be computed. For example, the rotational constant of the diatomic molecule can be explicitly expressed via the average interatomic distance (Equation ).