the channel for the single-layered system, {\tt gmx make_ndx -n index.ndx -twin} will
provide you with the groups for the double-layered system.
+To suppress large fluctuations of the membranes along the swap direction,
+it may be useful to apply a harmonic potential (acting only in the swap dimension)
+between each of the two channel and/or bilayer centers using umbrella pulling
+(see section~\ref{sec:pull}).
+
\subsection*{Multimeric channels}
If a split group consists of more than one molecule, the correct PBC image of all molecules
with respect to each other has to be chosen such that the channel center can be correctly