The development of three physics-based energy functions (force fields), designed to simulate the restricted free energy of proteins of the alpha, beta, and alpha/beta structural classes. is described. Each force field corresponds to a particular weighting of the united-residue (UNRES) interactions defined in earlier work.(1-6) To find the optimal weights for the alpha, beta, and alpha/beta force fields. both the Z-score and energy gap of the native versus normative structures are minimized simultaneously for four benchmark proteins: 1pou for the (alpha force field), 1tpm (for the beta force field), and 1bdd and betanova (for the alpha/beta force field). The simultaneous minimization was carried out by using a novel Monte Carlo method, Vector Monte Carlo (VMC). For alpha -helical proteins, another weighting of the UNRES interactions (denoted as the ao force field) was developed; this fourth force field is described in a companion publication (Lee, J. et al. J. Phys. Chem. B 2001, 105, 7291). The structural implications of the final weights of the four force fields, i.e., the relative contributions of the various UNRES interactions to stabilizing common structural motifs of proteins, are analyzed. The alpha (0), alpha, beta, and alpha/beta force fields were used in the CASP4 exercise for ab initio protein -structure prediction with reasonable success. Finally, using a simple model system it was shown that the VMC protocol does not require exhaustive sampling of medium- and high-energy structures in order to optimize the parameters of the potential energy adequately.