Table . Calculated ΔGQB with different ligand force fields (PFROSST and GAFF) and experimentally measured koff and respective KD values for the HSP90α Vernalis dataset. Data were obtained from Schmidtke et al., and via personal communication by James Murray, Vernalis Ltd.; values correspond to triplicates, except those shown in parentheses, which correspond to single measurements. Hence, alchemical methods have become very popular for drug design. In the best case, they can be quite accurate (within 1 kcal mol -1 of the experimental value). However, accuracy decreases for challenging transformations (e.g., change in topology, net charge, multiple atoms, or protein conformation).