  We propose a novel method for refining force-field parameters of protein
systems. In this method, the agreement of the secondary-structure stability and
instability between the protein conformations obtained by experiments and those
obtained by molecular dynamics simulations is used as a criterion for the
optimization of force-field parameters. As an example of the applications of
the present method, we refined the force-field parameter set of the AMBER
ff99SB force field by searching the torsion-energy parameter spaces of $\psi$
(N-C$^{\alpha}$-C-N) and $\zeta$ (C$^{\beta}$-C$^{\alpha}$-C-N) of the backbone
dihedral angles. We then performed folding simulations of $\alpha$-helical and
$\beta$-hairpin peptides, using the optimized force field. The results showed
that the new force-field parameters gave structures more consistent with the
experimental implications than the original AMBER ff99SB force field.
