Before discussing in more detail the vibrational spectra of H 2 Cl + and its isotopologues, we have checked the convergence of the energy levels with respect to the basis set and the grid sizes. In our tests, the three 1D-basis sets have the same number of basis functions, nb which increases from 9 to 12 and the grid point number, nq, is fixed to 12. These tests have shown, that, the largest error between the energy levels up to 4000-5000 cm -1 calculated with nb = 11 and nb = 12 is smaller than 10 -2 cm -1 . Therefore, we have chosen 12 basis functions for each Harmonic oscillator basis and for all isotopologues.