Several approximations are employed in AutoDock, in order to make the calculations suitable for rapidly docking large libraries of compounds. Scores are single point evaluations of the scoring function, thus neglecting the majority of entropic contributions. Bond lengths and bond angles are treated as rigid and non-polar hydrogens are omitted. Rotatable bonds are allowed to rotate freely, neglecting rotamer preferences. Partial charges are based on the empirical method from Gasteiger and Marsili , and can be considered "lower quality" in comparison to charge assignment methods based on some form of electronic structure calculation, even via semi-empirical methods such as AM1-BCC .