The CYANA target function (Güntert et al., 1991; Güntert et al., 1997) is defined such that it is zero if and only if all experimental distance restraints and torsion angle restraints are fulfilled and all non-bonded atom pairs satisfy a check for the absence of steric overlap. A conformation that satisfies the restraints more closely than another one will lead to a lower target function value. The CYANA target function for distance restraints and torsion angle restraints is defined by
Upper and lower bounds, bαβ, on distances dαβ between two atoms α and β, and restraints on individual torsion angles θi in the form of allowed intervals are considered. Iu, Il and Iv are the sets of atom pairs (α,β) with violated upper, lower or van der Waals distance bounds, respectively, and Ia is the set of restrained torsion angles. wu, wl, wv and wa are overall weighting factors for the different types of restraints, and and wi are relative weighting factors for individual restraints. The half-width of the forbidden range of torsion angle values is denoted by , and Δi is the size of the torsion angle restraint violation.
The target function may include additional terms for restraints on vicinal scalar coupling constants, residual dipolar couplings, and pseudocontact shifts, as well as identity and symmetry restraints for symmetric multimers. Alternatives to the simple square potential for violated distance restraints have also been implemented.