where R A m and Z A m label the coordinates and atomic number of the m th atom of A, R B m and Z B m label the m th atom of B, and the operator ∇ 2k is shorthand notation for k consecutive applications of the Laplace operator. Despite its simplicity, in our previous study we found this model to reproduce water dimer interactions to a fair degree of accuracy, albeit, with a rather high approximation order (six or higher).