Cooperativity and Stability in a Langevin Model of Protein Folding
  We present two simplified models of protein dynamics based on Langevin's equation of motion in a viscous medium. We explore the effect of the potential energy function's symmetry on the kinetics and thermodynamics of simulated folding. We find that an isotropic potential energy function produces, at best, a modest degree of cooperativity. In contrast, a suitable anisotropic potential energy function delivers strong cooperativity. 
Gabriel F. Berriz, Alexander M. Gutin, and Eugene I. Shakhnovich   (Harvard University)