Carl Troein Enumerating Designing Sequences in the HP Model Master Thesis Advisor: Anders Irbäck Abstract: The hydrophobic/polar HP model on the square lattice has been widely used to investigate basics of protein folding. In the cases where all designing sequences were enumerated without restrictions on the number of contacts, the upper limit on the chain length has been 18-20 because of the rapid exponential growth of the number of conformations. We show how a few optimizations push this limit by about 5 units. Some statistics are done on the designing sequences, and we compare with what happens when maximal compactness is forced by reduction of conformational space. December 2000 LU TP 00-51