The magnitude of the error would be expected to increase with the number and strength of restraints, and decreasing volume of the flat-bottom region. This is likely to be particularly problematic for systems where there are substantial changes in the conformations of the lig-and and the binding site upon binding. In addition, the use of average positions assumes that ∆G Distort. = 0 and U r, Rigid (x, y, z, ψ, θ, ϕ) = W r (x, y, z, ψ, θ, ϕ). This produces a slight bias towards more positive free energies of binding, because a flexible system will distort to minimise the sum W r (x, y, z, ψ, θ, ϕ) + ∆G Distort., Point (x, y, z, ψ, θ, ϕ) ≤ U r, Rigid (x, y, z, ψ, θ, ϕ) for a given relative position and orientation. ∆G Distort., Point (x, y, z, ψ, θ, ϕ) is related to ∆G Distort.