However, some terms of this sum can be zero because points Q n can be out of the action range from the current value q (those that fulfill the inequality (q -Q n ) 2 > 2σ 2 , see Eq. 4). Thus, we can order the Q n by their proximity to q , i.e., in the increasing order of q -Q n , and take only those Ñ ≤ N values that correspond to nonzero contributions to the potential (i.e., (q