  Perturbing a Go model towards a realistic protein Hamiltonian by adding
non-native interactions, we find that the folding rate is in general enhanced
as ruggedness is initially increased, as long as the protein is sufficiently
large and flexible. Eventually the rate drops rapidly towards zero when
ruggedness significantly slows conformational transitions. Energy landscape
arguments for thermodynamics and kinetics are coupled with a treatment of
non-native collapse to elucidate this effect.
