I agree with time plus Feliway. One of my cats, Olive, hid in a closet and vomited daily for a week when we first got Sweep. We didn't know if we'd be able to keep Sweep. Feliway plug-ins and catnip helped a lot. Olive still kept her distance for several weeks, but within a few months she was rolling around on the floor, belly exposed, just a few inches away from Sweep. You won't see them all cuddled up together at my house, but they do coexist peacefully.