code: private Vector3d getReferenceAxisCylic() {
  		if (rotationGroup.getPointGroup().equals("C2")) {
  			Vector3d vr = new Vector3d(subunits.getOriginalCenters().get(0));
  			vr.sub(subunits.getCentroid());
  			vr.normalize();
  			return vr;
  		}
  
  		// get principal axis vector that is perpendicular to the principal
  		// rotation vector
  		Vector3d vmin = null;
  		double dotMin = 1.0;
  		for (Vector3d v: principalAxesOfInertia) {
  			if (Math.abs(principalRotationVector.dot(v)) < dotMin) {
  				dotMin = Math.abs(principalRotationVector.dot(v));
  				vmin = new Vector3d(v);
  			}
  		}
  		if (principalRotationVector.dot(vmin) < 0) {
  			vmin.negate();
  		}
  
  		return vmin;
  	}
docs: Returns the default reference vector for the alignment of Cn structures
  @return