code: public static final Atom unitVector(Atom a) {
  		double amount = amount(a) ;
  
  		double[] coords = new double[3];
  
  		coords[0] = a.getX() / amount ;
  		coords[1] = a.getY() / amount ;
  		coords[2] = a.getZ() / amount ;
  
  		a.setCoords(coords);
  		return a;
  
  	}
docs: Returns the unit vector of vector a .
  
  @param a
  an Atom object
  @return an Atom object