Finally, we examine the computational cost in q-AQUA-pol for both energy and gradient calculations for a 256-water system, using one single core or multiple cores of 2.4 GHz Intel Xeon processor, where the cost for each correction term ∆V n-b is also listed. The 256-water structure is chosen from MD simulation of liquid water using our recently developed water potential, q-AQUA. Note, the number of terms given is less than the factorial result owing to the use of the finite range switching function. For example, the total number of 3-b interaction terms in this 256 mer water system is . Overall the speed up is a factor of 5.9.