To investigate the convergence with respect to the size of the polarizable inner region, we used reference values calculated using an inner-sphere radius of 24 Å and kept the outer region at a constant size with a radius of 30 Å in all calculations. In Fig. , we show the absolute differences of the frequencies with increasing sizes of the inner region. We find that already at 8 Å the maximum errors are below 3 cm -1 , and they drop to below 1 cm -1 at 14 Å. In Fig. and, we present the normalized absolute and relative differences for the IR and Raman intensities. Here, we see that there is only one case where both the absolute and relative errors are notable at the same time, namely the Raman intensity of normal mode 11 when using an inner-sphere radius of 8 Å. However, by increasing the sphere size, the errors go down substantially. Since the overall intensity errors are small and to keep the maximum errors well below 1 cm -1 , we chose a 16 Å radius for the inner sphere.