When there is absolutely no phase information at hand there are two main methods used for the location of heavy atoms in the unit cell. The most commonly used is the Patterson method by which the heavy atom structure is found by deconvolution of the auto-correlation function of the heavy atoms electron density. The second is direct methods which has been found useful since the heavy atom structure consists of only a few atoms and is discussed in a separate section (Sec. ). If some initial phase information is available the presence of heavy atom sites may be confirmed or new weaker sites may be found by the isomorphous or anomalous difference Fourier technique. These three methods will now be described in turn.