The protein chain is represented by all heavy atoms plus backbone hydrogen atoms. Bond lengths and bond angles are held fixed at ideal values, with the exception of the tau-angle (N-CA -C), which is allowed to vary in the range 108 to 114 °. Atom radii are as follows: carbon with double bond character = 1.50 Å, all other carbon = 1.70 Å, nitrogen = 1.35 Å, oxygen = 1.34 Å, cysteinyl sulfur = 1.70 Å, methionyl sulfur = 1.80 Å, hydrogen bonded to backbone nitrogen = 0.9 Å, hydrogen bonded to CA = 1.0 Å.