GoldScore fitness function is the original GOLD scoring function and it is selected by default. It is made up of four components: protein-ligand hydrogen bond energy and van der Waals energy, ligand internal van der Waals energy and ligand torsional strain energy. Optionally fifth component ligand intra-molecular hydrogen bond energy may be added. Empirical parameters used in fitness function such as hydrogen bond energies, atom radii and polarizations, hydrogen bond directionalities etc. are taken from a parameter file.
Goldscore function uses bond strengths in the fitness function, which is of form
,
where Shb_ext is the protein-ligand hydrogen bonding score, and Shb_int the internal hydrogen bonding of the ligand. Usually, the best result is obtained by letting the internal hydrogen bonding tend to zero. Svdw_ext and Svdw_int are the scores arising from weak Van der Waals forces.
Goldscore has a mechanism for placing the ligand in the binding site, which is based on fitting points. The program adds hydrogen-bonding fitting points to the protein and ligand. Then it maps acceptor points on the ligand on donor points in the protein and vice versa. Additionally, it generates hydrophobic fitting points in the protein cavity onto which ligand CH groups are mapped. The fitness function in GoldScore is optimized for the prediction of binding positions rather than binding affinities.
The actual search algorithm is a genetic algorithm optimizing several parameters of which one is the fitting point score described above. Other parameters are dihedrals of ligand rotable bonds, ligand ring geometries, and dihedrals of protein OH and NH3+ groups. It is obvious, that all the variables arise from the multiplicity of possible conformations the molecules can be stretched into.