A method for calculating the shifts of vibrational excitation frequencies in molecular clusters is presented. It is based on second order non-degenerate perturbation theory and refers to early publications of Buckingham. The resulting formula involves off-diagonal cubic force constants thus accounting for coupling of the individual molecular normal modes. It is applied to the C-O stretching mode in methanol dimers and to all three modes in water dimers. The results are in good agreement with experimental values with exception of the ν₃-mode of the donor molecule in the water dimer where the perturbation approach was found to be not adequate.