where l is the number of terms, T k (t, a j , r) is term number k and its weight is w k . t is one of the special types for ligand atoms involved in the term calculation. The special type is used to speed up the calculation of the scoring function for each position of the molecule (this is because the types may be repeated for some ligand atoms). These types denote characteristics of atoms, e.g. van der Waals radii.