A "converse" stability condition is necessary for a compact higher order scheme on non-uniform meshes

The stability bounds and error estimates for a compact higher order Numerov-Crank-Nicolson scheme on non-uniform space meshes for the 1D time-dependent Schr"odinger equation have been recently derived. This analysis has been done in $L^2$ and $H^1$ mesh norms and used the non-standard “converse” condition $h_{\omega }\le c_0\tau$, where $h_{\omega }$ is the mean space step, $\tau$ is the time step and $c_0>0$. Now we prove that such condition is necessary for some families of non-uniform meshes and any space norm. Also numerical results show unacceptably wrong behavior of numerical solutions (their dramatic mass non-conservation) when this condition is violated.