code: public static final Atom[] centerAtoms(Atom[] atomSet, Atom centroid)
  			throws StructureException {
  
  		Atom shiftVector = getCenterVector(atomSet, centroid);
  
  		Atom[] newAtoms = new AtomImpl[atomSet.length];
  
  		for (int i =0 ; i < atomSet.length; i++){
  			Atom a = atomSet[i];
  			Atom n = add(a,shiftVector);
  			newAtoms[i] = n ;
  		}
  		return newAtoms;
  	}
docs: Center the atoms at the Centroid, if the centroid is already know.
  
  @param atomSet
  a set of Atoms
  @return an Atom representing the Centroid of the set of atoms
  @throws StructureException