The accuracy of calculated vibrationally-averaged (usually B 0 ) rotational constants is mostly determined by two factors, (a) equilibrium structure and (b) vibrational corrections B e -B 0 . The latter is relevant for the context of the present investigation. Thus, in the following we do not aim at reproducing the absolute values of rotational constants but benchmark only corrections B e -B 0 . For this, five small molecules has been chosen (see Figure ): water (H 2 O), ethylene (C 2 H 4 ), difluoromethane (CH 2 F 2 ), trans and cis isomers of 1,2-difluoroethylene (C 2 H 2 F 2 ). For each molecule the corrections B e -B 0 were calculated by using Wigner sampling (in WS+MD and SWS+MD variants) and by VPT2. In our case with small semi-rigid molecules the values from VPT2 provide accurate reference values. Three of the test molecules (H 2 O, C 2 H 4 , and CH 2 F 2 ) were used to verify the position of optimal τ . Figure illustrates the effect of τ in the SWS procedure applied to these molecules. The minimal vibrational energies were in the range of 2-4 fs, very close to optimal τ = 2 fs determined for H + 2 . Thus, we used this value further in our simulations. However, in general case we recommend to tune τ for obtaining the most accurate results.