Here is a restraint which applies a singleton score to a list of particles. There is also a singleton score which acts on the distance to a fixed point. This restraint should probably replace the SphericalRestraint.

%pythonprepend ListRestraint::ListRestraint %{ args[1].thisown=0 %} %pythonprepend DistanceToSingletonScore::DistanceToSingletonScore %{ args[1].thisown=0 %}