The contact information is further assisted with secondary structure potential functions. This function is a residue wise statistical potential energy function that drives the /–w torsion angle combination of the residue into favorable conformations (Figure 1c). The function is prepared from a population analysis of 18,352 X-ray structures in Protein Data Bank (PDB) [22] with under 2.0-Å resolution. The local secondary structures of each residue are assigned with DSSP program [23], and eight residue types of DSSP are classified into three simpler secondary structures: helix, sheet, and coil. The population of the /–w angle combinations is transformed to energy values to make two-dimensional potential functions, assuming they satisfy Boltzmann distribution.